freshwater prawn culture

Louis R. DAbramoProfessor

Department of Wildlife and FisheriesThad Cochran National Warmwater Aquaculture Center

Mississippi State University

Cortney L. OhsResearch Associate II

Department of Wildlife and FisheriesThad Cochran National Warmwater Aquaculture Center

Mississippi State University

Mack W. FondrenResearch Associate II

Thad Cochran National Warmwater Aquaculture CenterMississippi State University

James A. SteebyAssistant Professor/Extension Aquaculture Specialist

Thad Cochran National Warmwater Aquaculture CenterMississippi State University

Benedict C. PosadasAssistant Research and Extension Professor

Coastal Research and Extension CenterMississippi State University

For more information, contact Dr. DAbramo by telephone at (662) 325-7492 or by e-mail [email protected]. Bulletin 1138 was published by the Office of Agricultural Communications, a unit of theDivision of Agriculture, Forestry, and Veterinary Medicine at Mississippi State University.

Culture of Freshwater Prawnsin Temperate Climates:

Management Practices and Economics

Mississippi Agricultural and Forestry Experiment Station 1

Commercial production of freshwater shrimp orprawn (Macrobrachium rosenbergii) has been the sub-ject of research and commercial enterprise in theUnited States for several decades. This species is nativeto the tropical Indo-Pacific region of the world. Basicproduction techniques were developed in the late 1950sin Malaysia, and in the United States, Israel, and sev-eral Asian countries during the last three decades. In1984, the Mississippi Agricultural and Forestry Ex-periment Station (MAFES) initiated an extensiveresearch program to develop and evaluate managementpractices for the establishment of a commercial fresh-water prawn industry as a supplement or alternative tothe culture of channel catfish, an established industry.

This bulletin is based upon the results of researchefforts during the past 16 years and provides a detaileddescription of the management practices for the threephases of culture (hatchery, nursery, and pond growout)of the freshwater prawn in temperate climates. Impor-tant information concerning the biology of the speciesas well as supplies and equipment necessary for suc-cessful culture is also presented.

The prospects for an economically successfulprawn industry in certain regions of the United Stateshave increased dramatically because of the develop-ment of improved management practices that have beensuccessfully applied to commercial production sys-tems. These production practices, for the first time,efficiently manage the unique biology of the prawn.

INTRODUCTION

GrowthFreshwater prawns, like all crustaceans, have a

hard outer skeleton or shell that must be shed (molting)regularly for growth to occur. Increases in body weightand length of the prawn principally occur soon aftercompletion of each molt. Growth is therefore incre-mental rather than continuous.

BreedingFemales generally become reproductively mature

by 6 months of age. Mating occurs only between hard-shelled males and soft-shelled females (i.e., maturefemales that have just completed a molt). The maledeposits sperm held within a gelatinous mass under-neath the body of the female between her fourth pair ofwalking legs.

Within a few hours after mating, spawning occursthrough the release of eggs that are fertilized by thesperm and then transferred and attached to the under-side of the abdomen (tail) in a brood chamber formedby the abdominal swimming appendages. The eggs areaerated and cleaned by movement of these appendagesand remain attached to the abdomen until they hatch.Mating and spawning can occur in either freshwater orbrackish water.

As long as water temperature exceeds 21C (70F),multiple spawns per female can occur annually. Thenumber of eggs produced at each spawn is directly pro-portional to the size of the female. Females carryingeggs are often termed berried females.

The rate of egg maturation and eventual hatchingincreases as water temperature increases. At a tempera-

LIFE HISTORY

Culture of Freshwater Prawnsin Temperate Climates:

Management Practices and Economics

2 Culture of Freshwater Prawns in Temperate Climates: Management Practices and Economics

ture of 28C (82.4F), the eggs hatch approximately2021 days after spawning. The bright yellow color ofnewly spawned eggs gradually changes to orange, thenbrown, and finally to gray about 23 days before hatch-ing. Newly hatched freshwater prawns then enter into alarval phase.

LarvaeWhen larvae are released after hatching, they swim

upside down and tail first. The larvae cannot survive infreshwater beyond approximately 48 hours and requirebrackish water for growth and development to con-tinue. In the wild, hatched larvae are transported downrivers to brackish coastal water. Larvae are very aggres-sive sight feeders that feed almost continuously, andtheir natural diet primarily consists of small zooplank-ton, large phytoplankton, and larval stages of otheraquatic invertebrates. Larvae undergo 11 molts, eachrepresenting a different stage of metamorphosis prima-rily characterized by changes in the morphology (form)of the body. Following the last molt, larvae transforminto postlarvae. The duration of time necessary fortransformation from a newly hatched larva to postlarvadepends upon quantity and quality of food, tempera-ture, light, and a variety of other water qualityvariables.

PostlarvaeAfter metamorphosis to postlarvae, the prawns

resemble miniature adults, having a total body length of710 mm (0.30.4 in) and weighing 69 mg(50,00076,000 prawns per pound). During the earlypart of the postlarval phase, the behavior of the prawnschanges from free-swimming and inhabiting the watercolumn (pelagic) to crawling principally and inhabitingthe bottom (benthic). When swimming does occur, themovement is adult-like with the dorsal (back) sideup in a head-forward direction.

In the natural environment, postlarvae tolerate awide range of salinities and eventually migrate up riverto freshwater. In addition to the food they ate as larvae,their diet now includes larger pieces of both animal andplant material such as larval and adult aquatic insects,algae, mollusks, worms, fish, and feces of fish andother animals.

Postlarvae are translucent, and as they grow, theygradually take on the bluish-green to brownish color

characteristic of the adult stage. The term juvenilegenerally refers to individuals that are several weeksbeyond postlarvae in age but have yet to reach the adultstage. However, no standard definition for the juvenilestage exists.

AdultOlder juveniles eventually enter into the adult stage

and usually have a distinctive blue-green color,although sometimes they may take on a brownish hue.Color is influenced by the quality of diet. Identificationof adult males and females is easily accomplishedthrough examination of the ventral (bottom) midbodyregion of the prawn. The base of the fifth or last pair ofwalking legs (periopods) of males is expanded inwardto form flaps or clear bubbles that cover the openings(gonopores) through which sperm are released. In fe-males, the gap between the last pair of walking legs ismuch wider and a genital opening is located at the baseof each of the third pair of walking legs.

Three different forms (morphotypes) of adult maleshave been identified based upon external and physio-logical characteristics. Easily distinguishable are theblue claw (BC) males that are characterized by long,spiny blue claws. Two other male morphotypes areorange claw (OC) and strong orange claw (SOC). OCmales sequentially transform to SOC and then to BCmales, but the actual conditions that cause transforma-tion after the occurrence of a molt are not completelydefined. Some OC males in the population are charac-teristically smaller than other males (often referred toas small males) in the population due to comparativelyslower rates of growth. Although these males are small,they are reproductively mature and play a greater rolethan other OC males in reproductive activities.

Males that mate with females are restricted to BCmales and some of the smaller OC males that are repro-ductively mature. Only the BC male maintains aterritory associated with a group of females that areready for mating. He protects this harem of females,particularly during a postmolt period when they arevulnerable to cannibalism. As the age of BC malesincreases, reproductive capacity diminishes. BC malesundergo an extended period of nonmolting (anecdysis)when no growth occurs. Eventually, the BC male willmolt and return to a growth phase during which itsreproductive capacity is gradually renewed.


Procurement of SeedstockProduction of juvenile freshwater prawns for stock-

ing into growout ponds begins with maintenance of ahealthy broodstock population. In temperate climates,prawn broodstock are generally selected from the cropharvested from ponds and then transferred to tanks orraceways located within a temperature-controlled build-ing. Water temperature for broodstock holding shouldrange between 25C and 28C (77F and 82.4F).Broodstock are generally stocked at a density of 19prawns per liter (1.15 oz/gal) in a ratio of 10 females to23 males. If you plan a 4- to 5-month holding periodbefore collection of egg-bearing females for larval pro-duction, then you must include 34 OC males for everyBC male. Feed broodstock a high-quality diet contain-ing at least 35% crude protein; a high level of energy, 3kcal/g (85 kcal/oz); and at least 0.5% highly unsaturatedfatty acids (a commercially availablesalmonid fish diet would be suitable).The feeding rate should be equivalentto 13% of the prawns body weightper day. Divide that amount of feedinto two separate feedings of equiva-lent amounts. Equip tanks or racewaysthat hold broodstock with structuresthat allow maximum use of the entirewater column. A few weeks before theeggs are near hatching, feed brood-stock a supplemental beef liver at anequivalent ration on a dry weight basis(moisture content of beef liver = 80%).Cut frozen beef liver into half-inchpieces and rinse with water to removeexcess blood that might cause foulingof the system.

A mature female produces approximately 500 eggsper gram of live body weight (14,000 eggs per ounce).At the previously stated recommended range of holdingtemperature, normal egg development is characterizedby a series of color changes from bright yellow toorange to brown to a gray-green. Gray-green eggs gen-erally hatch within 2472 hours. To remove femalesthat hold eggs that are about to hatch, partially drainholding tanks and directly transfer selected females tospecial hatching tanks (Figure 1) that contain water ofsimilar temperature. Salinity of the water in thesehatching tanks should be 05 g/L (parts per thousand[ppt]). Larvae usually hatch from eggs at night and areattracted to light. Place a low-intensity light above theoverflow pipe of the hatching tank to attract the larvae,and they will flow into a separate, adjoining collectiontank. Position a small mesh screen 90l20 m

MANAGEMENT PRACTICES

Hatchery/Seedstock

The three phases of freshwater prawn culture arehatchery, nursery, and pond growout. Initial planningand operation of a prawn production enterprise shouldtemporarily forego the hatchery phase and possibly thenursery phase. Although the hatchery and nurseryphases are comparatively shorter, future investment oftime and money should be based on achieving successrepeatedly in the pond growout stage. Any plans for de-velopment of a nursery and possibly a hatchery phase

of production should be approached with careful plan-ning. Nursed juveniles for stocking into growout pondscan be purchased from a supplier. A limited number ofsuppliers of juvenile prawns currently exists, and anincreased demand will lead to the establishment ofmore enterprises that exclusively produce postlarvaeand juveniles (analogous to producers of fingerlings forstocking production ponds in the catfish industry).

Hatching tank

BroodstockLarvae

Larval collector Biological filter

Overflow pipe with screen

Mesh screen

Light

Air lift

Figure 1. Design of larval hatching and collection unit (fi = water flow).


(3.5x10-5 to 4.7x10-5 in) around the overflow pipe ofthe collection tank to prevent larvae from escaping.Water from the collection tank then flows either intoanother tank or returns to the hatching tank.

On the following day, determine the concentrationof larvae (number per liter) in the collection tank. Then,remove the appropriate number of larvae for stockinginto tanks for the hatchery phase of culture. The recom-mended initial stocking density for hatchery culture is5080 larvae per liter (189300 per gallon). Larvaeshould be collectively stocked only from hatches thatoccurred within a 1- to 3-day interval. Before stocking anew batch of larvae, feed the previously stocked larvaeso that they have at least partially full guts. This proce-dure minimizes the incidence of cannibalism oflate-stocked larvae by larger larvae that were stockedearlier, and it ensures that a narrow range of larvalstages exists at any time during the culture period.Maintenance of a narrow range of larval stages (sizes)also minimizes the duration of the harvest of postlarvae.

Culture ConditionsLarval culture must be conducted in tanks that

receive indirect sources of natural light with intensityequivalent to a typical late morning or early afternoonon a partly cloudy to clear day. During the early morn-ing and late afternoon, complement the natural lightwith intense artificial light. However, never use artifi-cial light as an exclusive substitute for natural light.

For larval culture, we recommend clearwater(minimal algal growth) recirculating systems (Figure 2)with water at a temperature of 2830 C (82.486 F)and a salinity of 1215 g/L (ppt). Use of recirculatingsystems, whether locatedinland or along a coast,provides for efficient useof water and reduction ofheating costs. Water in thelarval culture system ispumped from a collectingreservoir (sump) through asand filter, and thenthrough an ultravioletlight unit and a biologicalfilter before it enters intothe larval culture tank(Figure 2).

The biological filter isrequired to remove certain

nitrogenous waste products (ammonia, nitrite) that can betoxic if allowed to accumulate to sufficiently high con-centrations. Biological filters contain a high-surface-areasubstrate (media) upon which living bacterial populationsgrow and oxidize ammonia (the principal waste productof larval prawns) to nitrite and then to nitrate (a nontoxiccompound). The biological filter should contain approxi-mately 6% of the volume of the entire culture system, andthe rate of water flow through it should be 30100% ofthe volume of the entire system per hour. Newly hatchedlarvae stocked at the highest recommended density (100per liter) will require the highest flow rates (70100% ofthe total water volume per hour).

The sand filter should contain 850-micron sandparticles that serve to remove particulate matter fromwater efficiently before the water flows through theultraviolet light unit and the biological filter. Removalof particulate matter from the water increases the oper-ational efficiencies of both the ultraviolet light andbiological filter. Exposure to ultraviolet light dramati-cally reduces the concentration of bacteria in the water,including pathogenic bacteria. The sand filter must beflushed (backwashed) once to several times daily,depending upon the size of the larvae (resident biomassin the system) and the amount of food fed. This proce-dure is designed to prevent accumulation of particulateorganic material that can clog or cause channeling ofwater flow, which would reduce the filtering efficiency.Other types of systems designed for the removal ofparticulate material from water in recirculating systemsare available. Daniels et al. (1992) have provideddetails concerning requirements for materials andequipment based upon a specific production goal.

Figure 2. Design of a larval culture unit.


Clean, sterilize, and flush the larval culture systembefore adding water. Water used for the initial fillingshould pass through a 5-micron bag filter. After the sys-tem is filled and operational, add a chlorine-basedsterilizing agent to achieve a chlorine concentration of5 mg/L (parts per million [ppm]). If you perform thissterilization procedure several days before stockingnewly hatched larvae, then no dechlorinating agents(i.e., sodium thiosulfate) are required. This protocol isrecommended because the presence of dechlorinatingagents has been implicated in causing mortality ofprawn larvae. If only freshwater or slightly saline wateris available, then you must add a commercially pro-duced salt mixture and thoroughly mix it with thefreshwater to achieve the appropriate salinity for cul-ture. Always use high-quality marine salt mixtures ofproven effectiveness because inferior salt mixtures canadversely affect growth and even cause mortality.

Preparation and Maintenanceof Biological Filter Media

The water volume of the biological filter should beat least 6% of the total volume of the culture tanks thatit serves. A variety of materials can serve as biofiltermedia (surface material). However, the media mustprovide a large surface area for the growth of bacterialpopulations. Either all or a portion of the biologicalmedia should be calcareous material such as smallcrushed oyster shell or coral. Storage and handling ofmedia are facilitated when it is contained in bags fash-ioned from fiberglass window screen.

Media destined for use in the biological filter are acti-vated in a separate preconditioning container byintroducing other media that already have established andgrowing populations of nitrifying bacteria. Once ap-propriately conditioned, selected quantities of the biofiltermedia are transferred to the actual biological filter unit asneeded (i.e., as the biomass of the larvae and their corre-sponding rate of ammonia production in the culturetank(s) increase). Temperature (2830 C [82.486 F])and salinity (12 g/L [ppt]), should be the same in both theactivating and the culture tanks. Constant, vigorous aera-tion must be supplied to the activating tank. Following isthe procedure for activation of media for use in a biologi-cal filter, adopted from Daniels et al. (1992):

(1) Calculate the expected daily maximum load ofammonia-nitrogen in the larval culture system basedon the anticipated number of postlarvae to be pro-

duced in the entire larval system. Based on empiri-cal data, the maximum rate of production ofammonia-nitrogen (ammonia-N) by a larva of M.rosenbergii in a closed, recirculating culture systemis approximately 30 g per day (1.05x10-6 oz perday). For example, if the maximum expectedamount of ammonia-N produced by 2 million larvaewithin the system in a 24-hour period is 60 g (2.12oz), then 226.8 g (8 oz) of ammonium chloride needto be oxidized completely by the biofilter mediabeing activated in the preconditioning tank. Thisdetermination is based upon that fact that 1 g (0.035oz) of ammonium nitrogen exists in 3.78 g (0.133oz) of ammonium chloride. A bag of crushed coralweighing 2.26 kg (4.98 lb) usually serves as sub-strate for a population of nitrifying bacteria that iscapable of nitrifying (oxidizing) 1 g (0.035 oz) ofammonium chloride in 24 hours. Therefore, 227bags of crushed coral would be needed to nitrify 60g (2.11 oz) of ammonia-N. The maximum volume ofcoral media, representing less than 4% of the totalrearing volume, is generally reached by the 17th dayof a culture period or when the larval stage index ofthe population is 8.5 (Griessinger et al. 1989).

(2) Initially, add 10% of the total required ammoniumchloride (NH4Cl) or another inorganic source ofammonia to the water containing the media.

(3) After a few days, check the levels of total ammonia-N and nitrite-nitrogen (nitrite-N). Low-rangeammonia (0.00.8 mg/L [ppm] ammonia-N) andnitrite (0.00.2 mg/L [ppm] nitrite-N) test kits forsalt water are satisfactory for such determinations.If both levels are below detection, then add the sameamount of ammonium chloride recommended instep 2. If, however, either total ammonia or nitrite isstill detected, do not add any additional ammoniumchloride and recheck levels after 24 hours.

(4) Continue to add the recommended amount ofammonium chloride (see step 2) and check the lev-els of ammonia-N and nitrite-N. When this amountof ammonium chloride is completely nitrifiedwithin 24 hours, double the amount, and follow thepreviously stated procedure.

(5) As each increasing level of the introduced sourceof ammonia is consumed within the desired 24-hour period, double the amount of ammonia addedas ammonium chloride until the maximum requiredload is consumed daily (i.e., within 24 hours).

Generally, 2.26 kg (4.98 lb) of crushed coral mediacontaining a satisfactory population of nitrifyingbacteria will nitrify (oxidize) 1 g (0.035 oz) ofammonium chloride in 24 hours.

(6) After oxidation of the maximum level of ammoniais achieved within 24 hours, the larval culture cyclecan begin. As needed, the proper amount of mediais sequentially removed from the preconditioningtank and placed into the biological filter. Media thatremain in the preconditioning tank must still bemaintained at their maximum level of ammoniaand nitrite consumption. The amount of ammoniathat needs to be added for maintenance willdecrease as the amount of media in the precondi-tioning tank decreases.

Feeds and FeedingNo nutritionally complete, formulated diet is cur-

rently available to achieve consistently successfullarval culture of M. rosenbergii. Therefore, live food isrequired. Newly hatched nauplii of Artemia (brineshrimp) have been the overwhelming choice for use asa nutritionally complete diet. Artemia are available ascysts (dormant unhatched eggs) from a variety of com-mercial sources. Newly hatched Artemia with anundigested yolk sac are an excellent source of nutrition.After the cysts have been sterilized and fully or par-tially decapsulated, they should be hatched under cleanconditions to prevent newly hatched nauplii from beinga potential source of disease organisms when added tothe larval culture tank. A suggested procedure to pro-duce live Artemia nauplii for feeding follows:

(1) Cyst hydration Cysts are hydrated by immer-sion in fresh or seawater (less than 35 g/L [ppt]) at25C (77F) for 1 hour.

(2) Sterilization and decapsulation Cysts are thensterilized and decapsulated through the addition of1 g of commercial calcium hypochlorite (HTH) perliter of hydration water. Cysts should be kept in thissterilizing bath for 20 minutes. During the decap-sulation process, the cysts should be kept awayfrom direct sunlight.

(3) Washing and deactivation Cysts are separatedfrom the bath by pouring the mixture through a120-micron (0.0047-inch) screen. Cysts that arecollected on the screen are then thoroughly washedwith freshwater or seawater until the odor of chlo-

rine is no longer detected. Toxic chlorine residuesthat may adsorb to the decapsulated cysts can bedeactivated by two dips into a 0.1 N hydrochloricacid (HCl) or acetic acid (CH3COOH) solution asrecommended by Bruggeman et al. (1980). Theduration of the deactivation should not exceed 30seconds, and it should be followed by anotherwashing of the cysts.

Hatching of cysts is best achieved in conical-bottom,funnel-shaped, PVC containers that are equipped with avalve at the narrow end. Stock cysts at approximately 1.5g/L (0.20 oz/gal) in natural or artificial salt solutions hav-ing a salinity of 1012 g/L (ppt). The hatching water canbe enriched with 2 g/L (ppt) of sodium bicarbonate(NaHCO3). The pH of the water should remain above 8,and water temperature should be within the range of2530 C (7786 F). Provide aeration to ensure that lev-els of dissolved oxygen are maintained above 2 mg/L(ppm). Illuminate the hatching containers with 60-wattfluorescent light bulbs (1,000 lux) that are located 20 cm(7.87 in) above the water surface. After approximately24 hours, harvest hatched Artemia nauplii according tothe following procedure:

(1) Turn off air; remove standpipe (if one is used),heater, and airstones. Then, cover the top of thehatching container with a dark lid or black plasticfor 1520 minutes. Unhatched cysts and shells fromhatched cysts will rise to the surface and have a darkbrown color. Artemia nauplii are bright orange, andmost should concentrate within the water columnnear the bottom of the hatching container.

(2) Slowly drain the water containing the newlyhatched nauplii from the bottom of the containerthrough a l20-micron (0.0047-in) mesh screen andstop when the dark brown Artemia eggshells beginto appear.

(3) Thoroughly rinse the nauplii collected on thescreen with fresh or brackish water.

(4) Nauplii newly hatched from a total of 50 g of cystscan be safely stored in 1 L of seawater held withinan insulated container and chilled to not less than5C by the addition of ice packs. The reduction inwater temperature caused by this procedurereduces the metabolism of the nauplii and the rateof loss of nutrients from the yolk sac, thereby sus-taining the highly nutritional value of the food.


Generally, 150,000 Artemia nauplii hatch from 1 g ofcysts. However, the hatching characteristics (rate, hatch-ability) of cysts vary according to time, storageconditions, geographical origin, and commercial brand.Exercise caution when purchasing cysts. A comparativelylower purchase price generally indicates lower hatchingperformance, etc., and the cost-effectiveness of use of thislower quality must be considered. Generally, the poorperformance of some batches will not be adequately com-pensated by a reduced selling price. Most prawn larvaebegin feeding 1 day after hatching (larval stage 2). It isbetter to provide frequent feedings of live food from sun-rise to sunset, rather than one or two feedings spread overa long interval of time. Without frequent feedings, thenutritional value of uneaten Artemia in the water columndecreases over time because the nutrients contained in theyolk sac are continually being removed to satisfy growthand metabolic needs.

Newly hatched, live Artemia nauplii retained on a120-micron-mesh harvest screen are fed to prawn larvae.A suggested daily feeding rate of nauplii according to day

poststocking and stage index is presented in Table 1. Theinitial morning feeding should consist of 40% of the totalnumber of Artemia to be fed that day (daily ration), fol-lowed by 20% of the ration later in the morning. Theremaining 40% of the daily ration is fed during the after-noon. Any excess Artemia that remain after the dailyration has been fed should be frozen in cubes within icecube trays. This procedure is recommended as a safetyprecaution for use during the morning of the followingday if a sufficient amount of live Artemia are not avail-able due to a poor hatch, or simply for use as an initialearly-morning feeding.

No later than midmorning, collect a sample con-sisting of 50100 larvae and examine them under adissecting microscope to determine whether their gutsare full. Full or mostly filled guts indicate healthy indi-viduals. During the entire larval cycle, be careful tomonitor routinely whether guts are full. Empty oralmost empty guts are an indicator of inferior cultureconditions such as poor water quality, high levels ofbacteria, or insufficient levels of food provided.


Table 1. Stage-dependent feeding rates for Artemia nauplii and for the supplemental diet. Recommended particle size of supplemental diet and the mesh size of the screen for flushing are included.

Day of Stage Artemia per larva Supplemental feed Particle Flushing cycle index a.m. p.m. Upper Lower size screen

no. no. mg mg m m1 1 0 0 - - -2 1.5 3 3 - - -3 1.8 3 3 - - -4 2.2 9 8 - - -5 2.7 10 9 - - -6 3.2 12 10 - - - 3007 4.0 16 14 (0.08) (0.08) 300-5008 4.8 22 20 (0.09) (0.08)9 5.4 27 23 (0.11) (0.11)

10 5.6 32 28 (0.18) (0.15)11 6.4 38 32 0.3 0.2 500-700 50012 6.9 42 38 0.38 0.2513 7.2 47 43 0.43 0.314 7.9 49 45 0.55 0.415 8.3 51 47 0.65 0.5 700-900 70016 8.9 53 48 0.75 0.617 9.1 54 51 0.8 0.618 9.6 54 51 1.1 0.6 900-120019 9.8 56 54 1.2 0.7520 1st 58 58 1.2 0.8

Postlarvae21 65 65 1 0.822 58 58 1 0.923 58 58 0.85 0.924 56 56 0.85 0.825 53 53 0.75 0.7PL 62 62 0 0.3

Supplemental FeedA supplemental inert diet is usually fed during mid-

morning and late afternoon, approximately 710 daysafter a postlarval production cycle begins. The guts ofthe larvae should be as full of Artemia as possiblebefore feeding of the supplemental diet. When supple-mental feeding occurs, position a large-mesh screen(150, 400, or 710 microns, depending upon the size ofthe larvae) around the standpipe of each culture tank toallow for the exit of uneaten or partially eaten Artemiaand feces from the tank. The ingredient composition ofa typical supplemental diet is fish or squid, chickeneggs, beef liver powder, and a marine fish oil that is agood source of highly unsaturated fatty acids (Table 2).A recommended procedure for the preparation of a sup-plemental diet follows:

(1) Thaw squid or fish at room temperature or in amicrowave oven. Clean squid by removing pen, inksac, skin, eyes, and beak; clean fish by removingscales, skin, and bones. Sterilize the squid or fishby placing it in a microwave oven and cooking it onhigh for 78 minutes per kilogram (3.18 minutesper pound). Homogenize the sterilized fish or squidtissue in a commercial-grade food processor until awell-blended mixture (i.e., smooth texture with nochunks) has been achieved.

(2) Mix chicken eggs, marine fish oil, and beef liverpowder together well, and then add this mixture tothe squid or fish homogenate within the foodprocessor.

(3) Gradually add an ingredient for binding purposes(e.g., alginate) and continue mixing slowly until apaste eventually forms and then begins to formballs and detaches from the walls of the foodprocessor.

(4) Take the paste and form thin patties manually orwith a press. Then place these diet patties into aplastic bucket containing approximately 45 g/L(ppt) of calcium chloride (CaCl2). A slightly addi-tional amount of CaCl2 can be added to the water toincrease the rate of binding. The outer layer of eachpatty will begin to harden quickly and eventuallydevelop a rubbery texture. When this change in tex-ture has occurred, press a patty between your handsand then slide your hands in opposite directions toproduce a thinner patty. After the patties have beenseparated and have assumed a rubbery texture, they

are processed in a food mill. As larvae increase insize during the production cycle, replace the foodmill with a 1.6-mm (l/16-in) cheese grater to pro-duce larger particles of the diet. Create smallerparticles by manually pushing the material throughsieves to obtain the desired particle sizes.Suggested mesh sizes are 250-micron (0.009-in),425-micron (0.017-in), 600-micron (0.024-in),850-micron (0.033-in), and l,000-micron (0.039-in). The resulting sieved diet should be rinsedthoroughly to remove fine particles that can foulthe water and contribute to unwanted bacterialgrowth within the culture tank. Drain the feedbefore storing either refrigerated (several days) orfrozen. The size of particle fed depends on the sizeof larvae; it normally ranges from 2501,000microns (0.0120.039 in) (Aquacop 1977).

Separation of Larvae and PostlarvaeAfter metamorphosis through the 11 larval stages

has been completed, larvae then metamorphose intopostlarvae. After a significant proportion of larvae(2533%) has transformed to postlarvae, transfer theremaining larvae to another culture tank so that the post-larvae can be collected for transfer to the nursery phaseof culture. The relocated larvae will eventually transformto postlarvae. Generally, two transfers of larvae arerequired per production cycle. Separate larvae from post-larvae during mid- to late morning after postlarvae haveeaten and are clinging to the wall of the culture tank,while larvae are localized in a feeding ring away fromthe wall of the tank. Collect larvae from these areas ofconcentration with a small-mesh net and move them toanother tank. Be careful to ensure that water quality inthe transfer tank is the same as that in the culture tank. Inround cylindrical tanks, larvae and postlarvae can beeffectively separated by creating a vortex of water at thecenter of the tanks through the use of paddles. Free-swimming larvae are concentrated within the watercolumn at the center of the tank while postlarvae cling tothe sides and bottom of the tank.


Table 2. Ingredient composition of supplemental diet.

Ingredients Percent wet weight

Squid, cleaned 85Cod liver oil 2Eggs 10Beef liver powder 3

After transferring the larvae, transport one-half totwo-thirds of the water in the tank where the postlarvaeremain to another holding tank and sterilize this waterfor future use. The postlarvae are now ready for accli-mation to freshwater. Freshwater should be addedgradually, so that salinity eventually decreases to 0 pptwithin a 24- to 36-hour period. At the end of thisperiod, determine the mean weight of individual post-larvae by weighing a bulk sample of a known number

of postlarvae. This procedure will provide an estimateof the number of postlarvae produced per productioncycle. The desired number of postlarvae to be stockedinto each tank (density) used in the nursery phase canbe accurately monitored by dividing the total biomass(weight) of groups of postlarvae to be stocked by themean individual weight. Generally, survival at termina-tion of the hatchery phase of culture ranges from4080%.

The nursery stage of culture is the period when juve-niles are produced for stocking into production ponds.This management practice is included for culture of M.rosenbergii in temperate climates to increase an other-wise time-restricted growing season due togrowth-limiting and lethal water temperatures in produc-tion ponds. A by-product of this management approach isa larger animal for stocking into growout ponds, whichreduces the potential for poststocking mortality due tostress or predation by insects.

Nursery culture is generally conducted in tanks withinclimate-controlled buildings. Water temperatures shouldrange from 2528 C (78.882.4 F). The design of a nurs-ery facility will vary according to the respective need forinsulation to maintain desired water temperature. In someregions, heated greenhouses may be sufficient, but otherlocations will require heated buildings that are insulated.The costs of maintaining the desired optimal water tem-peratures for growth during the nursery phase areimportant components in the assessment of the economicfeasibility of this phase. In most locations, immersionheaters will also be required to maintain water temperature.To conserve water and heat, water recirculation systems arerecommended. Flow-through systems equipped withheaters may also be used, but practicality is dependent onavailability, temperature, and cost of the water. The use ofrecirculating systems will require the activation and main-tenance of populations of nitrifying bacteria (biologicalfilters) to transform toxic ammonia to nontoxic nitrate.Development, use, and maintenance of biological filtersare described in the hatchery section of this bulletin, andthe same procedures described for brackish water systemsare applicable to freshwater systems. No pesticides shouldbe used in or near (at least 100 yd) a nursery facility.

The depth of nursery tanks/raceways for culture gen-erally should not exceed 1.2 m (4 ft) to provide for easymaintenance. Tanks constructed of a variety of plasticsand aboveground swimming pools with a liner of at least

0.1 mm thickness are suitable. Distribute artificial habitat(substrate) throughout the water column to increase theavailable surface area to permit prawns to distribute them-selves in three dimensions within the tank. As a result ofthis separation, the frequency of aggressive encountersand the opportunity for cannibalism are reduced. Theproducts of this management practice are an increase insurvival as well as a potential increase in the amount ofenergy allocated to growth. Include substrate at a level thatincreases the available surface area of the bottom andsides of the tanks by approximately 50%. An amount thatexceeds 100% of the surface area does not provide anyadditional benefit. If a flat material is used, then both sidesshould be included in the calculation of the amount of sub-strate required. During the period of the nursery phase, therequired amount of substrate can be added gradually asjuvenile prawns become larger and more aggressive.Growth or survival does not appear to be affected bywhether the substrate is oriented horizontally or verticallyin the water column (Wilson et al. 2002).

The stocking density for nursery tanks should rangefrom 36 postlarvae per liter of water (1223 postlarvaeper gallon). Stocking density can also be based upon theamount of substrate present in the nursery tank 215430 postlarvae per square meter (2040 per squarefoot) of surface area of substrate (Taylor et al. 2002). Thisrecommended stocking density based upon surface areaof substrate is similar to that based on water volume whenthe amount of substrate is equivalent to 50% of the com-bined surface area of the bottom and sides of a culturetank. The addition of substrate is critical. A 25% increasein survival was realized after 60 days of nursery culturewhen substrate was provided (Taylor et al. 2002).

The suggested initial stocking density is based uponachieving good survival and a suitable stocking size withinan economically practical amount of time. Typically, a nurs-ery phase of 4060 days results in a population of juvenileprawns with a mean weight of 0.10.3 g, individual weights


Nursery

that range from 0.40.8 g each,and survival that ranges from5580%. Survival varies accord-ing to stocking density, amountof substrate used, feeding rate,water quality, duration of thephase, and numerous other vari-ables. Under the density andassociated conditions prescribed,survival can be reasonably esti-mated by assuming 1.5%mortality per week for the first 4weeks, 3% mortality per weekfor the next 5 weeks, and up to3% per day for the size of juve-niles attained after 9 weeks.

During the nursery phase ofculture, the biomass of a popula-tion of juvenile prawns in a tank reaches a sufficiently highlevel whereby the juvenile prawns respond by a reductionin growth rate. This density-dependent growth responsehas been shown to begin when a biomass density of 0.5 g/Lis achieved (DAbramo et al. 2000). At a density of 5 juve-niles per liter, this density is achieved when the meanweight of the population is 0.1 g (100 mg). Therefore,under the prescribed stocking densities and other manage-ment protocol for nursery culture, growth rates are likelyless than optimal after the first 35 days of a 60-day nurseryphase. However, despite this density-dependent reductionin growth rate, the suggested densities are still designed toprovide for a cost-effective enterprise.

Prawns in the nursery phase should be fed a high-protein (approximately 55% crude protein, dry weight)trout or salmon starter diet (#1 then switch to #2 particlesize). Commercial diets manufactured by ZeiglerBrothers, Inc., (www.zeiglerfeed.com) or Rangen, Inc.,(www.rangen.com) have been used successfully. Duringthe nursery stage of culture, the particle size of diets fedshould not exceed #4. A feeding schedule based on per-cent of live body weight and an empirically derivedgrowth curve for M. rosenbergii during the nursery phaseof culture is provided in Figure 3. Divide the total dailyration into at least two separate morning and afternoonfeedings. To avoid poor water quality caused by over-feeding, adjust the amount of daily ration based upon theobserved consumption of food.

Three times per week, feed prawns a dietary supple-ment consisting of shredded frozen beef liver at a rateequivalent (on dry weight basis; liver moisture content is

approximately 80%) to the daily ration of formulated feed.The liver diet is best prepared by shredding it from a frozenform manually with a cheese grater. By following this pro-cedure, the liver particles can be either rinsed or slowlyintroduced directly to a tank so that uneaten particles do notaccumulate on the substrate or bottom of a tank. To avoida potentially rapid deterioration of water quality, dividefeeding of the total shredded beef liver ration into equiva-lent amounts over at least three separate times during theday. Despite this precaution, feeding beef liver in recircu-lating systems can somewhat compromise water qualityand partial water exchanges may be necessary.

Before stocking nursery tanks, calculating feedingrates, stocking ponds, or selling juvenile prawns, youmust first estimate weight. A relatively accurate estimateof a mean individual weight can be achieved by collect-ing several samples of at least 100 prawns, spinning themin a net to remove excess water, and then weighing them.Survival of prawns is not adversely affected by the spin-ning procedure. The calculated mean individual weightcan then be multiplied by the number of the prawnsdesired, and this total weight can be used to guide in thecollection of the actual number of prawns required.Samples are often disproportionately composed ofsmaller prawns that are easier to collect. As a result, cal-culation of the mean individual weight for the populationis often an underestimate, leading to an overestimate ofthe number of juveniles. This is a fundamental problem inobtaining an accurate determination of the amount ofjuveniles for sale to producers, and survival at the termi-nation of the nursery phase of culture.


7

9

11

13

15

1 2 3 4 5 6 7 8 9

Week

Per

cen

t o

f b

od

y w

eig

ht

0

1000

2000

3000

4000W

eigh

t gain

(% in

crease)

Figure 3. Feeding rate (percent of body weight) and growth rate of postlarvaeduring indoor nursery phase of culture. Postlarvae initially stocked at five to sixper liter.

= Feeding Rate

= Weight Gain

Size Grading of Nursery PopulationsSize grading of juveniles from a nursery-grown

population before stocking into production ponds is aneffective method to increase mean individual weightand total yield at harvest. Size grading is a simple stockmanipulation procedure commonly practiced in thehusbandry of terrestrial animals. Grading separates thelarger, fast-growing prawns from the smaller, slow-growing ones, a size disparity that is the product of thetypical social hierarchy that develops among males dur-ing the nursery phase. When these separatedpopulations are independently transferred to productionponds, growth of the smaller males is no longer nega-tively impacted by the presence of the larger,faster-growing males. After stocking into productionponds, the growth rates of smaller males commonly in-crease to compensate for the comparatively slowergrowth rates that occurred during the nursery phase(compensatory growth). The division of nursery-raisedpopulations by size results in a dramatic reduction inthe proportion of small males that is generally charac-teristic of prawn populations harvested fromproduction ponds stocked with ungraded juveniles. Thereduction in the number of small males at harvestincreases total yield and potential revenue. Theweighted production from ponds separately stockedwith each of two populations obtained by grading canbe 2530% greater than production in ponds stockedwith the same group of prawns that were not graded.However, recent research results suggest that theseincreases in production are not achieved when size-graded populations are stocked at a low density of21,000 prawns per hectare (8,500 per acre).

Size grading can be performed with either bargraders that are conventionally used to grade small fishor a derivation of the bar grader design. The type ofnumerical separation by size achieved will dependupon the bar width used and the weight (size) distribu-tion of the population of nursery-raised prawns.Experience has demonstrated that a good relationshipexists between bar width and the mean weight of thelargest prawns that pass through the bars in a verticalplane. A prawn size (weight)-bar width relationshipshould be determined for the specific size-grading tech-nique used. A 50%-50% (upper-lower) or 40%-60%(upper-lower) numerical separation is advised so thatcomparable numbers of juvenile prawns representingeach graded population are available for stocking. Becareful to avoid a situation where the result of the grad-

ing is a disproportionate number of prawns in one sizeclass (i.e., 80%). Stocking of populations arising froma 70%-30% (upper-lower) separation has still producedsubstantial increases in overall production relative tothat of ungraded populations that were stocked.

No specific grading procedure is recommended.Juveniles move toward a flow of water, and this behav-ior may assist in the use of passive grading techniques.Other, more active grading techniques would involve themovement of a grader through a population or the forcedmovement of a population through a stationary grader.The choice of technique should be based upon the expe-rience, ease, effectiveness, and resources available to theculturist. Always conduct size grading with the provisionof plentiful aeration to avoid stressful conditions.

Transport of Postlarvae and JuvenilesTwo methods are commonly used for shipping

freshwater prawn postlarvae and juveniles. The firstmethod of shipment is identical to that used for manyyears for live shipment in the ornamental fish trade.Either postlarval or small juvenile prawns are placedinto a plastic bag containing water and pure oxygen; thebag is placed in a cardboard box with a Styrofoam liner.This method is used for either airfreight or short-dis-tance ground delivery. The second method of transportis live haul and requires a tank/container with well-aer-ated (oxygen, or forced air) water. Agitators should notbe used to aerate the water because they will injure orkill postlarval prawns. Live-haul containers may ormay not be insulated. Live haul is a much more eco-nomical approach to transport comparatively largenumbers of postlarvae and juveniles that need to beshipped long distances. Live haul is also the only cost-effective method for transport of nursed juveniles thatare 30 days and older.

Some practices for successful shipment are com-mon to both methods. Prawns need to be acclimatedslowly to the conditions (temperature, salinity, pH, etc.)of the shipping water. Water for shipping is usuallycooled to within a range of 1822 C (6472 F) toreduce the level of activity and metabolism of theprawns specifically oxygen consumption and ammo-nia excretion. Shipping temperature should be basedupon the anticipated ambient temperature conditionsduring the time interval between shipment and receipt.The air temperature of vehicles, airline cargo holds, andloading docks en route, combined with duration ofexposure, will influence water temperature at the final


Postlarvae or juveniles for the pond growout phasecan be purchased through commercial hatcheries cur-rently located in several states, including Mississippi,Texas, Florida, Kentucky, and Tennessee. Stocking ofjuveniles is recommended to reduce poststocking mor-tality and control size variation at harvest. The pricevaries according to age (size) and quantity desired butis approximately $2030 per 1,000 postlarvae and$6085 per 1,000 juveniles.

Pond Design and PreparationProduction ponds for freshwater prawns should

have many of the basic features of ponds used for theculture of channel catfish. A good supply of fresh waterand soil with excellent water-retention qualities areessential. Well water is the preferred water source forraising freshwater prawns. Collected runoff from a sur-rounding watershed or runoff from rivers, streams, andreservoirs can be used. However, the quality of thewater may be subject to adverse changes, and sufficientquantity (availability) for needs is unpredictable.

Whatever the source, the quality of water must be eval-uated for its suitability for culture before a site isselected. Some water quality characteristics consideredabsolutely necessary for good prawn growth include atleast 90 days of water temperatures greater than 20C(68F), pH that ranges from 7.08.5, and a water hard-ness that ranges from 15300 mg/L (ppm). Pondsshould not be constructed in areas that are subject toperiodic flooding. Before stream or river water entersinto ponds, it should be passed through a nitex screenwith a mesh diameter that does not exceed 300microns. This procedure should prevent the undesiredintroduction of fish and fish eggs into the pond.

Analysis of soils for the presence of pesticides isanother procedure that is essential before selection of asite. Many pesticides applied in the management ofrow-crop farming are toxic to prawns. Therefore, pondsshould not be constructed in contaminated soils, inareas that are subject to drift from agricultural sprays,or in areas exposed to runoff water that may be suscep-tible to pesticide contamination. Samples from water

destination. Prawns should not be fed for at least 12hours before shipping. Lack of food will reduce the rateof production of ammonia, a toxic excretory product ofprotein metabolism. Periodically, determine the meanindividual weights of groups of prawns throughout theprocedure of loading for shipment. This ongoing deter-mination of mean weight of the population ensuresgreater accuracy in the provision of the desired numbersfor shipment. Those prawns first removed (captured) bynet from a culture tank are typically the smallest.

When shipping prawns, it is very important to con-sider density and weight (biomass). Generally, 5,000new postlarvae are shipped in 2.5-gal (9.5-L) aquariumtrade shipping bags. Each postlarva weighs approxi-mately 0.01 g, so the weight (biomass) density is 20g/gal (5 g/L). If larger nursed juveniles are to be shippedin these shipping bags, then stocking densities must bereduced significantly. Nursed juveniles that weighapproximately 0.1 g each a tenfold increase in weightover a newly metamorphosed postlarva should bestocked at a density of 750 per 2.5-gal bag (30 g/gal orapproximately 7.5 g/L). The weight per gallon (or liter)shipped increases by 50% for larger prawns; however,density decreases by 85%. A study conducted by Coyleet al. (2001) using sealed bags with pure oxygen, boxcontainers, and juvenile prawns of 0.26 g 0.02 g indi-

cated that transport at 25 g/L resulted in lowest cost perindividual prawn. Water quality and survival data indi-cate that stocking densities greater than 10 g/L anddurations exceeding 8 hours in sealed containers mayresult in a deterioration of water quality and stressfulconditions for transported prawns.

Tanks/containers used for live transport can vary insize, shape, insulation value, and aeration capacities.Recommended biomass densities (grams per liter) forlive haul and boxed shipping are similar. Live haulcapacities for juveniles of approximately 0.30.4 g eachare approximately 33 g/gal (8.75 g/L). Therefore, youwould ship approximately 6570 juveniles per gallon atthis stage of development. Larger juveniles need to beshipped at lower densities. Density can be increasedslightly for live haul trips that are less than 2 hours. Awater salinity of 1 ppt is commonly used for live hauls.Salinities of 45 ppt would likely be beneficial, and thecost for the additional salts would be included in thetransport cost. As expected, lower stocking densitiesyield higher survival, especially on longer trips withlarger prawns. Live-haul shipments with lower-than-recommended-biomass densities will most likely ensurehigher postshipping survival, but this benefit must beweighed against the cost of transport per individualjuvenile.


Growout

sources intended for use in culture should also bescreened for pesticide contamination.

Local or regional offices of the Soil ConservationService can provide assistance in pond design and lay-out. The surface area of growout ponds should ideallyrange from 0.42.0 ha (15 A). Successful production inlarger ponds has been achieved, but the logistics of man-agement and harvest present some problems. Ideally, theshape of the pond should be rectangular, thereby provid-ing the opportunity to distribute feed across the entiresurface area of water. The bottom of a production pondshould be completely smooth and free of any potentialobstructions to seining. It should also be free of any deepdepressions where prawns will escape capture by seineor become stranded if a drain harvest is performed.

Ideally, ponds should have a minimum depth of 0.6m (2.15 ft) at the shallow end and slope to a maximumdepth of 1.21.5 m (3.934.10 ft). The slope of thepond bottom should allow for rapid draining and con-sist of a 4-in drop in elevation for every 100 ft of pondbottom. A smaller slope may contribute to the forma-tion of small depressions on the pond bottom whereprawns become stranded during a drain harvest. If adrain harvest is planned, then a slightly deeper (1015cm, 46 in) area of 4.66.1 m (1520 ft) should be con-structed around the drainpipe. During drain harvest, theprawns will concentrate in this area to provide for apractical procedure for removal. Alternatively, if theextent of the drainage fall allows, prawns can be col-lected in a net or basket placed in water on the outsideof the pond levee. If a pond is designed properly andthe drainpipe is free of obstructions, this methodrequires the least amount of labor.

Best results for draining and harvesting ponds with0.41.2 ha (13 A) of water surface have been realizedwith one 35- to 40.5-cm-diameter (14- to 16-in-diame-ter) drainpipe or two 20- to 25-cm-diameter (8- to10-in-diameter) drainpipes included in the design. Withthe flow capacity of these pipes, full draining of mostponds will occur within 2448 hours. If more pipes orlarger pipe diameters are used, then the draining timewill correspondingly decrease. Provision of at least twopipes also provides backup if one pipe should becomeobstructed. Draining of the final 0.9 m (1 ft) of watershould be sufficiently slow to allow time for prawns toeither collect within the in-pond catch basin or passthrough the drain for collection outside the levee. One25-cm-diameter (10-in-diameter) pipe is ideal fordraining the final 0.9 m (1 ft) of water. Some prawns

may still have to be removed from the pond bottom asthe final water drop may strand some in soft muds.

Collect soil samples at six different locations fromthe bottom of a newly constructed pond and mix themto make a composite sample. Place each sample in asoil-sample box available from county offices of theMississippi State University Extension Service andsend it to the MSU Extension Soil Testing Laboratoryor another soil-testing laboratory to determine pH. Ifthe pH of the soil is less than 6.5, perform an applica-tion of agricultural limestone to increase the pH to atleast 6.5, or preferably 6.8.

Provision of Additional Habitat (Substrate)Much research has been devoted to the evaluation

of the effect of substrate in production ponds (Tidwellet al. 1998, 1999, 2000). Substrate consists of any two-or three-dimensional material that can be added to fillthe water column and serve as additional habitat for theprawns. A design that allows easy introduction andremoval, as well as a material that will give multipleyears of use are recommended. Substrate material thathas been commonly used in research investigations isan orange PVC barrier fencing often found along theperimeter of construction sites. This material is UV-protected and has been used for at least 5 consecutiveyears without deterioration in quality. Other materialssuch as bird netting or old nets have also been used suc-cessfully. Cost and availability are importantconsiderations in minimizing the proportional contribu-tion of this material to overall operational costs.

Substrate should be suspended vertically in thewater column, and the surface area of both sides shouldbe equivalent to at least 50% of the bottom surface areaof the pond, estimated as being equivalent to the watersurface area. Reinforcement bars (rebars) are commonlyused to support the vertical substrate within the watercolumn, and one rebar is positioned approximatelyevery 25 ft along the substrate. Provision of habitat hasresulted in as much as a 25% increase in total produc-tion in experimental ponds. Generally, realizedincreases are between 10% and 15%. A comparableincrease in production has yet to be demonstrated incommercial production ponds containing substrate.

Pond ManagementA feeding-fertilization program at or before stock-

ing similar to that used for catfish fry ponds isrecommended to discourage growth of common prob-


lem weeds such as Chara sp andNajas sp. After the ponds arefilled with water and at least 12weeks before the stocking of theprawns, apply an inorganic fertil-izer to shade out the growth ofunwanted (nuisance) aquaticplants. A liquid inorganic fertil-izer either 10-34-0 or 13-38-0 gives the best results andshould be applied at a rate of 1.9L (1/2 gal) per surface acre. Toassist in this procedure, inoculateeach pond with water from pondscontaining already-establishedblooms of desired microscopicalgal species. Do not use inor-ganic fertilizers to stimulatephytoplankton growth to shadeout undesirable aquatic plants that have alreadyappeared. Inorganic fertilizers stimulate growth of bothrooted and filamentous (moss) nuisance plants.Maintaining a proper phytoplankton bloom will facili-tate proper feeding and harvest of freshwater shrimp.

To stimulate an abundance of natural food organ-isms for the prawns, perform multiple applications oforganic materials such as distillers dried grains andsolubles, cottonseed meal, or sinking catfish feed.Choice of fertilizer should be based on cost and localavailability. Start the organic fertilization program witha one-time application of cottonseed meal or sinkingcatfish feed at 200300 lb/A after a pond has beenfilled. Continue fertilization with a commercial catfishfeed or meal (finely ground or small pellet is best) at arate of 1520 lb/A on alternate days until application offormulated feed begins, usually 68 weeks poststock-ing. At stocking densities of 8,00024,000 per acre,organic fertilization throughout the growout periodappears sufficient to sustain natural food populationsfor achieving maximum growth.

Operating pond depth should range from 34 ftduring the growout period. Pond depth during the ini-tial stocking and the beginning of the growout period,when water temperatures are generally cooler, could beincreased to 45 ft to discourage the growth of aquaticrooted plants and filamentous algae.

Prawn production in ponds can be negativelyimpacted by the presence of fish that are potential pred-ators, as well as competitors for formulated feed and

natural food resources. Existing fish populations mustbe eradicated before stocking prawns. There are twooptions for fish eradication: (1) completely drain thepond; or (2) apply 3 pt (1.41 L) of 5% emulsifiablerotenone per acre-foot of water. Rotenone is also poten-tially toxic to prawns. After it is applied, rotenonebreaks down at a rate influenced by temperature, light,levels of dissolved oxygen, and alkalinity. Generally,you can stock prawns without concern for rotenone tox-icity 23 weeks after application.

Stocking of JuvenilesBefore stocking, acclimate juveniles to pond condi-

tions by gradually replacing the water where they arebeing held with water from ponds where they will bestocked. Replace at least 50% of the transport water.The temperature difference between the holding systemand the stocking ponds should not exceed 3C (5.4F).

To avoid stress and possible mortality caused bylow temperature, stock prawns when the early-morningtemperature of the pond water is at least 20C (68F)for several days. This management guideline shouldsignificantly reduce the risk of mortality of juvenileprawns that would occur due to a rapid decrease inwater temperature to 15.6C or less (60F or less)caused by unanticipated low air temperatures.

Juveniles preferably those derived from size-graded populations and weighing from 0.10.3 g(0.0030.011 oz) have been commonly stocked atdensities ranging from 24,70049,400 per hectare


Figure 4. (A) Profile of a fresh-water shrimp pond with catchbasin and optional outsidecatch basket. (B) Top view ofpond with arrows indicatingslope fall direction, catch

A

B

(10,00020,000 per acre). Lower stocking densities willyield comparatively lower total harvested weight pergrowout period but higher weight per individual prawn.The duration of the growout period is dependent on thewater temperature of the ponds and generally rangesfrom 120l50 days in central Mississippi. Prawns couldbe grown year-round possibly two crops per year if sufficiently warm water is available.

FeedingJuvenile prawns stocked into earthen growout

ponds at the previously stated densities initially satisfytheir nutritional requirements by consuming naturalpond biota, such as insect larvae and worms.Researchers have evaluated a variety of feeding prac-tices involving the provision of different nutrientsources at different times during growout. However, forthe range of stocking densities previously recom-mended, commercially available sinking channelcatfish feed (32% crude protein) has been determinedto be an effective diet throughout the growout phase.Recommended feeding rates are based upon estimatedsurvival, estimated consumption expressed as a percentof live body weight, and the mean weight of the popu-lation derived from sampling of ponds (Table 3).Weekly rates based upon density without samplingmust be developed eventually for practical use.

A large proportion of the feed is presumed to be notdirectly consumed by the prawns but rather to serve asa fertilizer (a direct source of nutrients for the popula-tions of natural food organisms). A 1% mortality ratewithin the pond population is assumed per week, and atthe end of the pond growout season, survival generallyranges from 6085% when proper water quality ismaintained through recommended management prac-tices (see water quality management section). Yieldstypically range from 6701,350 kg/ha (6001,200lb/A). Mean individual weight is inversely related toproduction and ranges from 2865 g or 3615 wholeprawns per kilogram (167 whole prawns per pound).

Water Quality ManagementWater quality influences the rate of growth of

freshwater prawns. Dissolved oxygen is particularlyimportant, and a good oxygen-monitoring program isnecessary. Because prawns live on the bottom (in pondswithout substrate), levels of dissolved oxygen shouldbe routinely monitored within the bottom 0.3-m (1-ft)depth of water. Oxygen levels at the surface can poten-tially be lower than those at the bottom.

Electronic oxygen meters are the most reliable andaccurate means to determine levels of dissolved oxygen.These meters are comparatively expensive and requirecareful maintenance and calibration to ensure goodoperating condition and the collection of accurate data.The need for an electronic oxygen meter increases as thequantity of ponds that need to be managed increases.

If an enterprise consists of only a few ponds, thenmonitoring of dissolved oxygen levels can be readilyaccomplished through use of a commercially availablechemical oxygen test kit that is generally equipped toconduct 100 independent tests. Water samples for dis-solved oxygen analysis can be collected from thelowest foot of the water column with either commer-cially available or individually fashioned devices.

Dissolved oxygen concentrations should always bemaintained above 3 mg/L (ppm). At concentrationsbelow this level, stressful conditions and eventuallymortality will occur. Levels of dissolved oxygen thatare substantially higher than 3 mg/L (ppm) are recom-mended because chronically lower levels of dissolvedoxygen throughout the growing season can markedlyimpact yields. Emergency aeration can be achieved byan aerator, a device that increases the rate of transfer ofoxygen from air to water. The type and power of theaeration device(s) will be principally determined by thesize and shape of the culture pond. Generally, aerationgenerated by 1 horsepower per surface acre of water isrecommended.

During the late-evening and early-morning hoursof summer months, when the water temperature ofponds generally exceeds 25C, dissolved oxygen mustbe monitored frequently (every 23 hours) becauserapid decreases in oxygen commonly occur. To assist inpredicting whether the level of dissolved oxygen willlikely decrease to either stressful or lethal conditions,record the level of dissolved oxygen an hour after sun-set and again approximately 2 hours later. Plot thesetwo readings on a piece of graph paper and connect thepoints with a straight line. By extending the line from


Table 3. Weight-dependent feeding ratesfor semi-intensive pond growoutof Macrobrachium rosenbergii.

Mean wet weight Daily feeding rate1

25 g 3%

1Percent of body weight. As-fed weight of diet/wet biomass of prawns x 100.

these two points over time, you can estimate the dis-solved oxygen concentration before daylight (56a.m.). Use this method cautiously because dissolvedoxygen levels do not always decrease at a constant rate.Therefore, late-evening or early-morning dissolvedoxygen determinations are strongly advised. Decisionsto provide emergency aeration should be based on ananticipated decrease in dissolved oxygen (DO) below 5ppm. Aeration of ponds for 24 hours each day willreduce the magnitude of diurnal DO fluctuations, butsuch a management practice may not always preventDO levels from decreasing to below 3 ppm. Given thedesign of an aeration device operating 24 hours, con-tinuous water circulation may be a natural by-product.Water circulation may beneficially influence growth,but such a response has not been unequivocally demon-strated in controlled experiments. PTO-drivenpaddlewheels are recommended for emergency aera-tion, especially in ponds in the recommended upperrange (more than 1 A) of surface area.

Specific information concerning other water qual-ity requirements of freshwater prawns is limited.Although freshwater prawns have been successfullyraised in soft water (57 mg/L [ppm] total hardness) inSouth Carolina, the shell is noticeably softer and maybe more susceptible to bacterial infection. To avoid thiscondition, water hardness should range between 50 and200 mg/L (ppm). In very hard water (i.e., 300 mg/L[ppm] or higher), reduced growth and lime encrusta-tions on the exoskeleton have been observed. Hardnessof pond water can be increased through an applicationof a calcium source such as agricultural gypsum or cal-cium chloride. The purity of gypsum varies (7098%)and generally is more readily available than calciumchloride. Assuming 100% purity, an application of 1.72mg of gypsum per liter of water (ppm) can achieve anincrease of 1 mg/L (ppm) in total hardness.

Nitrogenous CompoundsAt concentrations of 1.8 mg/L (ppm), nitrite has been

associated with mortality in hatcheries, but no definitiveinformation derived either experientially or experimen-tally about the toxicity of nitrite to prawns grown inponds is available. During the pond growout phase, highnitrite concentrations in ponds would not be anticipatedgiven the level of prawn biomass associated with the rec-ommended stocking densities and feeding rates. Levels ofun-ionized ammonia that exceed 0.1 mg/L (ppm) canadversely affect the growth and health of fish in ponds. At

concentrations of un-ionized ammonia as low as 0.26mg/L (ppm) at a pH of 6.83, 50% of the prawns in thepopulation died within 144 hours (Armstrong et al. 1978).Therefore, concentrations of un-ionized ammonia thatexceed 0.1 mg/L (ppm) must be avoided.

pH RequirementsA high pH can cause mortality directly by creating

a pH inbalance relative to the prawn tissue. It can alsocause mortality indirectly by causing a larger proportionof ammonia to exist in the toxic un-ionized form.Although freshwater prawns have been successfullyraised in ponds where a pH has ranged from 6.010.0, apH that remains within the 6.59.5 range is recom-mended. High pH values usually occur in water havinga total alkalinity of 0.550 mg/L (ppm), often stimulatedby the existence of a dense algal bloom. Adding lime tothe bottom soil of ponds that are constructed in acidsoils can help to minimize severe and possibly lethalfluctuations of pH that might occur during growout.

One management practice that has been imple-mented to mitigate rising pH in smaller ponds isperiodic flushing (removing) of the top 30.5 cm (12 in)of surface water to reduce the quantity of photosyn-thetic algae in the pond. However, this procedure is nota practical solution in large ponds, and the quality ofthe effluent (water discharged) may not meet standardsestablished by state or federal agencies. Another man-agement approach to avoid high pH is to spread organicmatter, such as corn grain or rice bran, over the surfacearea of the pond. The organic matter should be intro-duced gradually, over a period of 2 weeks, to achieveeventually a level of 13 kg/A (32 kg/ha). The decom-position of the organic material releases carbon dioxidethat helps to reduce pH. Careful monitoring of oxygenlevels must accompany this management procedure.Oxygen levels would tend to decrease substantially dueto the heavy oxygen demand arising from the decom-position of the organic material.

Despite following the recommendations for prepar-ing a pond for stocking, a dense growth of filamentousalgae may still occur in production ponds. Feeding andseining cannot be performed effectively under theseconditions. Introducing low densities of herbivorousfish shortly after stocking the prawns could be an addi-tional precautionary management approach.

Certain aquatic herbicides, particularly Aquathol Kand Hydrothol 191, at recommended rates of applicationhave successfully controlled algae growth without hav-


ing any adverse effect on survival or behavior of shrimp.Bioassays to determine survival responses to a varietyof herbicides are needed. Before any herbicide isapplied, always conduct a simple bioassay. Selecthealthy juvenile prawns and place them in several plas-

tic buckets filled with aerated pond water containingeither no algicide or algicide at the recommended appli-cation rate. After 24 hours, if prawns exposed to algicideexhibit mortality or unusual behavior suggesting stress-ful conditions, then the algicide should not be used.

Unlike marine shrimp, disease has yet to be identi-fied as a major problem affecting production offreshwater prawns. This attractive characteristic isprobably due to the comparatively lower amounts oftotal biomass in production ponds relative to marineshrimp enterprises. However, as stocking rate and bio-mass per unit area increase, the potential fordisease-related mortality correspondingly increases.Some prawns in a pond population may be afflictedwith shell disease that is bacterial in origin and clini-cally manifested by black spots on the outer shell(exoskeleton). Incidence is usually associated withphysical damage to the shell. However, the disease is

not lethal and is eliminated by the shedding of the oldshell and the production of a new uninfected shell. Attimes, algae or insect eggs may be found adhered to theshell. This condition is neither disease- nor stress-related but would adversely affect consumeracceptance. Maintaining the best possible conditionsfor growth will encourage molting so that this conditionis minimized or eliminated. Disease problems are mostprevalent during the hatchery phase of culture and gen-erally result from the proliferation of bacteria caused byan undesirably high organic load. Addition of oxolinicacid at 1 mg/L (1 ppm) is the recommended therapeu-tic treatment.

Growout of freshwater prawns in temperate cli-mates involves stocking seed (juveniles) followed by aperiod of 110140 days of growth, depending on geo-graphical location. Pond harvest should be completedbefore morning water temperatures reach 15.6C(60F). Prawns can tolerate water temperatures to atleast 12.814.4 C (5558 F) if the temperaturedecreases gradually over several days. When pondwater temperatures below 68F occur for a consider-able part of a 24-hour period, prawn growth rates areso low that keeping them in ponds for any extendedperiod will not increase production appreciably.

At the end of the growout season, prawns may beeither seine or drain harvested. For seine harvest, ponddepth (or water volume) should be decreased by one-third before seining. Prawns can be held in small meshlivecars and loaded with a crane-basket onto trucks.Those that remain after seining can be harvested bydraining the pond to concentrate them in a large rec-tangular bar pit (ditch) that is deeper than thesurrounding pond bottom. Prawns then concentrate therefor seine harvest. Water in the ditch needs to be well aer-ated. Some prawns may not collect in the ditch afterdraining and will have to be removed from the pond bot-tom manually.

Harvest by complete drain-down is labor savingand more efficient. It can be readily and effectivelyaccomplished if ponds are properly designed with asmooth bottom and a slope that will ensure rapid andcomplete draining. Highly effective harvests have beenachieved with properly constructed ponds becauseprawns living at water temperatures higher than 68Fwill follow the receding water and eventually travelthrough a drain pipe into a collecting device or smallcollecting pond, generally located on the outside of thepond levee. There, sufficient aeration should be pro-vided to the water to avoid stress and possible mortalityas harvested prawns become concentrated. Adequatepond bottom slope and rapid drainage are critical to theefficient harvest of freshwater prawns. Ponds with veryflat bottoms and small drains create many logisticalproblems relative to harvest.

Freshwater prawns are very hardy animals and donot die or diminish in quality when exposed to sunlightand soft muds for a short period of time. They can becollected in buckets or baskets and rinsed with cleanwater with few losses as long as they are not packed inextremely dense groups and not exposed to warm airtemperatures for more than 1520 minutes.


DISEASES

HARVESTING

Whenever possible, aeration devices for maintainingproper levels of dissolved oxygen should be located at thedeep end of the pond adjacent to the drain basin area tominimize the accumulation of sediment there. Otherwise,aerators placed at the shallow end of a pond may producedepressions that will strand prawns as they follow thereceding water during the drain harvest of a pond.

Selective harvest of large prawns by seining duringa period of 46 weeks before final harvest has beenpracticed with the intent of increasing total yield from apond during a growing season. Selection of the meshsize of the seine (12 in) will depend on the desired har-

vest size of the prawn. Selective harvest may also be ac-complished with properly designed traps. Prawns havebeen trapped using a wide array of traps traditionallydesigned for the harvest of crayfish. The reduction inpopulation density caused by a partial seine or trap har-vest results in an increase in the growth rate of thesmaller prawns that remain. Through selective harvest,a freshly harvested product is available over a longerperiod of time. Insufficient research has been performedto determine conclusively whether a selective harvestpractice is cost-effective relative to a traditional, singlebulk harvest at the conclusion of the growing season.

Other research (Tidwell et al. 1996) has shown thatunder exact management practices and growing sea-sons of comparable duration (110140 days), meanindividual harvest weight of prawns and total produc-tion have the potential to be greater at higher latitudesin the northern hemisphere. Within the confines of suf-ficiently long growout periods, this phenomenonappears to be the result of lower mean water tempera-tures during the growing season. A comparison of the

composition of the harvested populations at differentlatitudes suggests that the prolonged cooler waterdelays development of the ovary and egg production infemale prawns. Since sexual maturity of females isdelayed until later in the growing season, energy thatwould have been spent on reproduction is transferred togrowth. As a result, larger females are produced despitethe shorter growing season at higher latitudes.


LATITUDINAL DIFFERENCES

Production goals and harvesting practices should bedeveloped in response to the market. Without thisapproach, financial loss due to lack of adequate storage(holding) facilities or price variability is inevitable.Demand suggests that there are small but lucrative nichemarkets for large live prawns and heads-on prawns on ice.Other forms will probably have to enter and be competi-tive within the marine shrimp commodity market.Year-round distribution of this seasonal product willrequire freezing. An individually quick frozen (IQF) prod-uct both whole and headless is an attractive form forsupermarkets or restaurants. Block frozen is also an alter-native method of processing for long-term distribution.Recent studies show that whole prawns, harvested 24hours before exposure to the IQF process, have a shelf lifeof at least 1 year when stored at -18C (0.4F). Cold-waterimmersion can be used to thaw frozen prawns for imme-diate use. Any other thawing is restricted to refrigeratedconditions. Overall acceptability is maintained for frozenprawns allowed to thaw for up to 24 hours under theseconditions (Silva and Handumrongkul 1998).

Recent research conducted at the MississippiAgricultural and Forestry Experiment Station suggeststhat adult freshwater prawns can be successfully live-hauled for at least 24 hours at a density of 0.060 kg/L(0.5 lb/gal) with little mortality and no observed effecton exterior quality of the product. Transport underthese conditions requires the provision of oxygen to thewater. The prawns should be distributed vertically, ashomogeneously as possible, throughout the water col-umn, possibly in stacked shelves. This approachavoids potential mortality due to stress and localizeddeterioration of water quality from crowding on thebottom of the transport tank. Ideally, the temperature oftransport water should be 2022 C (6871.6 F) toreduce the activity level of the prawns, thus minimizingthe incidence of injury and water quality problems, par-ticularly ammonia accumulation. Researchers areinvestigating an alternative method for overnight trans-port of live freshwater prawns using a minimal amountof water.

PROCESSING AND MARKETING

The evaluation of the economic feasibility of pondgrowout of freshwater prawn in temperate climates ofthe United States was based upon a hypothetical com-mercial pond production system (CPPS) usingexperimental and commercial results derived from thepractice of current pond growout technology inMississippi. Costs and returns of CPPS were estimatesbased on recommended management practices, biolog-ical knowledge of the species, estimated input usageand prices, and established ex-vessel shrimp prices.The CPPS was then evaluated under different combina-tions of economic and biological scenarios. Theeconomic model used in this analysis incorporated pro-duction characteristics from experimental ponds andcommercial operations. The model estimated owner-ship costs of a hypothetical commercial farm.

Experimental productionresults indicated that 12-count,heads-on prawns could be pro-duced at 1,345 kg/ha (1,200 lb/A)in 0.1-ha and 0.06-ha (0.25-A and0.15-A) ponds stocked with 30-day-old juveniles at a density of49,420 per hectare (20,000 peracre). Commercial prawn enter-prises have produced yields of897 kg/ha (800 lb/A) when 0.8-hato 1.2-ha (2-A to 3-A) water sur-face ponds were initially stockedat a density of 34,595 juvenilesper hectare (14,000 juveniles peracre) (Posadas et al. 2001, 2002).Experimental results also indi-cated that at stocking densities of34,595 juveniles per hectare,1,065 kg/ha (950 lb/A) of 10-count prawns are produced in0.1-ha (0.25-A) and 0.06-ha(0.15-A) ponds. To simplifyassumptions, two stocking densi-ties, 20,000 and 14,000 juvenilesper acre (49,420 and 34,595 juve-niles per hectare), were used inthe economic analysis. The size ofprawn farms currently rangesfrom a few water acres to more

than 160 water acres. For the purpose of this analysis, ahypothetical 50-water-acre (20-ha) commercial opera-tion was assumed using 25 appropriately designed,2-water-acre (0.8-ha) production ponds. Tables 4-5present the critical biological and economic parametersused in the analysis of hypothetical, risk-free manage-ment systems for pond production of freshwater prawnat different levels of investment (dollars per acre). Theprincipal differences between the two pond manage-ment systems are stocking density (juveniles per acre),desired harvest size (number per pound), and expectedfarm-gate price (dollars per pound). As a consequenceof lower stocking density, ponds will yield fewer totalpounds of prawns per acre. However, the larger individ-ual prawns produced at a lower density can be sold at ahigher farm-gate price. Historical, monthly ex-vessel


ECONOMIC FEASIBILITY

Table 4. Critical model parameters and simulation results of hypothetical,risk-free freshwater prawn pond management systems with a stocking density

of 14,000 per acre under different levels of investment, Mississippi, 2002.

Critical parameters Scenario I 1 Scenario II 2 Scenario III 3and investment levels

Critical Biological ParametersStocking density (postlarvae/A) 14,000.000 14,000.000 14,000.000Survival (%) 75.000 75.000 75.000Desired harvest count (#/lb) 10.000 10.000 10.000Stocking size (g) 0.150 0.150 0.150Desired harvest size (g) 45.400 45.400 45.400Gross feed conversion 2.500 2.500 2.500

Critical Economic ParametersFarm-gate price, heads-on ($/lb) 4.000 4.000 4.000Processing yield, headless (%) 45.000 45.000 45.000Number of production ponds (#/farm) 25.000 25.000 25.000Size of production ponds (A/pond) 2.000 2.000 2.000Experimental-commercial yield gap (%) 92.000 92.000 92.000Capital outlay index (%) 100.000 100.000 100.000

Model DescriptionNumber of crops (#/yr) 1.00 1.00 1.00Operating capital ($/yr) 106,143 106,143 102,259Initial fixed investment ($) 266,704 215,279 143,733Number of juveniles stocked (M/yr) 0.70 0.70 0.70Average shrimp production (heads-on) (lb/A) 966.00 966.00 966.00Total shrimp production (heads-on) (lb/yr) 48,300.00 48,300.00 48,300.00Feed required (ton/yr) 60.38 60.38 60.38

Model ResultsNet return above specified expenses ($/yr) 44,588 49,580 64,573Payback period (yr) 3.910 2.941 1.782Net present value (10-yr cash flow) ($) 109,170 183,801 302,883Internal rate of return (10-yr cash flow) (%) 20.003 29.828 54.348

1Investment includes pond construction, all machinery, and all land.2Investment includes pond construction and all machinery, but no land.3Investment includes pond construction and aquaculture machinery, but no land.

prices of headless marine shrimpproducts in the northern Gulf ofMexico and monthly averageimport prices of frozen headlessmarine shrimp products pub-lished by the National MarineFisheries Service website(www.st.nmfs.gov/st1/) were usedas a basis for determining themost likely farm-gate prices for aheads-on, fresh, on-ice productsold in the prawn commoditymarkets. Another marketing formthat producers can consider is aheads-on and headless individu-ally quick frozen (IQF) product.

Determination of the eco-nomic feasibility of aninvestment project is based uponthe internal rate of return (IRR)method. The decision rule is thefollowing: if IRR is greater thanor equal to the cost of capital,then the project is accepted.Otherwise, it is rejected. The costof capital is the interest rate atwhich money can be borrowedand is based upon the prime rateat which banks borrow moneyfrom the Federal Reserve Banksystem. Another interest rate that can be used for com-parison to the IRR is an investors expected rate ofreturn, commonly accepted as 25%. This rate is higherthan the interest rates of commercial banks because therisk factor of the investment is included. The higherpercentage rate used in the decision rule represents amore conservative approach in determining whether toaccept or reject a project. The payback period (PP) esti-mates the number of years necessary to recover theinitial investment from the expected annual net incomebefore any allowance for depreciation.

Three different levels of investment adapted fromDAbramo et al. (2002) were used to evaluate the eco-nomic feasibility of the CPPS models. Scenario I refersto a CPPS that must invest in newly constructed 2-water-acre ponds, all farm and aquaculture machinery,and newly acquired land (Table 6). Scenario IIdescribes a CPPS with investment on new pond con-struction and all machinery but with land already

owned. Scenario III requires investment on new pondconstruction and aquaculture machinery only, whilegeneral farm equipment and land are already owned.

The base CPPS model labeled as Scenario Irequires an initial fixed investment of $266,704 (Tables4-5). A detailed description of the type, number, andcosts of land, pond construction, machinery, and equip-ment necessary for the base CPPS model is presented inTable 6. With a stocking density of 20,000 juveniles peracre, operating capital amounts to $124,510 per crop(Table 6). Given the base model assumptions, an esti-mated 57,500 lb of 12-count prawns are producedduring a 4-month growout period each year. The esti-mated average cost of production is $2.90/lb,consisting of $2.17/lb average variable costs and$0.74/lb average fixed costs. The major cost items arejuveniles (36%), feed (15%), labor (13%), and repairand maintenance (9%) for operations, as well as depre-ciation (56%) and interest on investment (37%) for


Table 5. Critical model parameters and simulation results of hypothetical,risk-free freshwater prawn pond management systems with a stocking density

of 20,000 per acre under different levels of investment, Mississippi, 2002.

Critical parameters Scenario I 1 Scenario II 2 Scenario III 3and investment levels

Critical Biological ParametersStocking density (postlarvae/A) 20,000.000 20,000.000 20,000.000Survival (%) 75.000 75.000 75.000Desired harvest count (#/lb) 12.000 12.000 12.000Stocking size (g) 0.150 0.150 0.150Desired harvest size (g) 37.830 37.830 37.830Gross feed conversion 2.500 2.500 2.5

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