Freshwater Prawns in Ponds

Freshwater prawns, Machrobrachium rosenbergii, or shrimp are currently popular with small scale aquaculturists in the southeasternUnited States.  Pond acreage has been constructed in nearly all states, but particularly inKentucky,Mississippi,Texas,Tennessee,Georgia, andAlabama.  Research atKentuckyStateUniversity has reported larger yields (Tidwell et al. 1999) than other states in the region (D’Abramo et al. 1998), reportedly due to the cool summer climate and reduced competition between reproductively active individuals.  Several other aspects of that research can lead to improved production over other systems attempted in the Southeast.  Utilization of water from a reservoir rather than a well can allow zooplankton to be seeded in prawn ponds prior to initial fertilization and larval prawn stocking.  The pH of a surface water reservoir can be lower than well water from calcium carbonate containing aquifers due to watershed geology and lack of alkalinity in rainwater that fills the reservoir.  The effective use of structure to supplement pond bottom surface area allows prawns access to their natural foods and refuge from aggressive individuals, especially during molting.

Prawn culture methods outlined by those promoting the growth of prawns assumes that natural food organisms are present when ponds are stocked with post-larvae or small juveniles.  Fertilization is recommended to produce blooms of algae and zooplankton, but many prawn producers fail to achieve the necessary zooplankton numbers to provide adequate food for the prawns.  Often, plastic tanks and plastic lined ponds fail to produce prawns because these artificial systems lack natural food organisms.  Earthen ponds will eventually establish zooplankton blooms if fertilization is applied, but the timing of prawn stocking must be delayed until a suitable bloom is present.

The system for prawn production involves three phases: hatchery phase, nursery phase, and grow-out phase.  Mature prawns from the grow-out phase are then used as spawners for the hatchery phase.  Hatcheries have developed inTexas,Mississippi,Kentucky,Florida,South CarolinaandHawaii.  Most are “high health” facilities with the promise of certified disease free stock.  However, there is always a concern that viral diseases may be encountered if due diligence is not practiced by the prospective prawn buyer.  The nursery phase has developed as a means to grow prawn post-larvae to the juvenile stage.  Producers who wish to lower their cost of seedstock have become distributors who grow post-larvae for up to 60 days then sell the larger juveniles to other producers.  They hope to buy larger quantities at reduced prices and assist other producers by reducing transportation costs.  The grow-out phase takes from three to five months depending on the size of prawn desired.  Ponds are stocked with 15,000 to 40,000 juveniles per acre in order to obtain 600 to 1,500 pounds of prawn by the end of the growing season. 

A good site for prawn production would be a site similar to that needed for construction of other types of aquaculture ponds.  The terrain should be flat to gently sloping but well drained.  Ponds constructed below the water table are not easily drained and would not allow efficient prawn harvesting.  The best site would have soils with at least 25% clay content within two feet of the surface.  However, pond liners may be utilized in soils with lower water holding ability.  Water availability is important since freshwater prawn ponds are filled each year.  The water source can be from ground water or surface water.  Location of freshwater prawn ponds near population centers is good for marketing.  The pond site should also be convenient to the manager’s center of operations for labor and security reasons. 

Pond construction planning must take into account engineering requirements, biological needs of the species that is being cultured, and cost of construction.  When exceptions are made in these areas, catastrophic results can occur.  Some examples of pond failures include: dam failure due to improper compaction or soil type, rapid sedimentation and levee erosion when levee slope is too steep, and business failure when pond construction costs are excessive for the potential return from aquaculture.  Experts must be consulted when constructing a pond in order to minimize the risk associated with pond construction errors.  The Soil Conservation Service, now the National Resource Conservation Service, has been the best source of this information in the past.  The Cooperative Extension Service can provide information about pond construction requirements for aquaculture.  Many county extension agents have a great deal of knowledge of pond construction.  The ultimate responsibility for pond construction decisions falls on the potential pond owner.  In large pond projects, hiring a licensed engineer may be the best decision.

Levee height and top width are variables that can improve the quality of the pond or add to the cost of pond construction.  Minimum engineering requirements are recommended as follows: Top width at least 8 ft for levees less than 10 ft tall, freeboard of at least 1 ft on levee ponds that are less than 656 ft long, levee slope of 3:1 (unstable soils require 4:1 to prevent excessive erosion) (Wheaton 1985).  Levee-type ponds require 1,100 to 1,200 yards of dirt moving per acre when built on level land (Wellborn 1988a).  Pond levee top width is recommended as 16 ft to allow heavy vehicle passage (Wellborn 1988b).   Remember, any increase in levee height must be added at the bottom of the levee, the widest part.  For example, a change from 6 ft  to 10 ft levee height adds at least 1,798 cubic yards of dirt per acre to the soil needs for building a pond.  When amortized over a 10 year period, the added cost of this additional pond depth would amount to an additional $300 to $450 per acre per year in cost of operation.  Similar relationships of smaller magnitude can be shown for increases in levee width or levee slope.

A source of electrical power is needed at each pond that will provide economical power to aerators.  Larger ponds may require a source of three-phase power in order to power larger aerator motors.  It is common to provide one to four horsepower of aeration per acre of water. 

Ponds used to culture prawns are generally narrow and usually about 50 ft in width.  They are also relatively small, a 0.5 to 1.0 acre size is common.  The narrow width is justified to allow more efficient feed distribution to all areas of the pond.  Boats must be used to distribute feed evenly over ponds that are wider.

Prawn ponds can also constructed with catch basins (Figure 1).  The rule of thumb specifies a catch basin of 10% of the pond area.  Side slopes of the catch basin follow the same rules as for the pond itself.  Erosion and sedimentation will eventually fill catch basins, proper construction slows this process.  The pond drain should be placed in the bottom of the basin.  A four inch drain may be adequate for 0.25 acre ponds, but eight or 10 inch drains should be used for 0.5 to 1.0 acre ponds (Tidwell et al. 2002).  Remember to allow for the depth of the basin and slope of the drain when laying out  pond elevations.  Most catch basins will be constructed 12 to 18 inches below the elevation of the pond bottom.  Since water quality in the catch basin will be poor, a source of freshwater near the basin will help during harvest. Flushingthe catch basin dilutes nutrients and suspended material so water quality can improve.  External catch basins can be used if drains are large and can be flushed with fresh water to move prawns through the drain.

A harvest basin can be constructed outside the pond (Figure 2) so that no internal catch basin would be needed.  In that case, a 10 or 12 inch drain line must be used.  Also, the pond must slope evenly to the drain.  The slope of the site must be adequate to allow rapid draining of water and prawns through the drain lines.  The drain slope should be 4- 5%.  Therefore, the top of the external harvest basin must be at least two feet below the elevation of the pond bottom.  Wire boxes or cages are used to trap the prawns as they exit the pond drain.  Captured prans are then moved to fresh water or chilling tanks.

The average depth of ponds should be no less than four feet.  Prawns do not like light.  Shallow areas expose the prawns to predators.  Light penetration into shallow water encourages aquatic weed growth that will make harvest difficult and will reduce pond productivity.

The addition of structure to prawn ponds can increase prawn yield.  Usually 50% of the surface area of the pond bottom is covered by structure.  Plastic construction barricade material serves the purpose of structure by being durable and having wide openings to allow feed, prawns, and water to pass through.  Structure should be horizontally suspended so it is about 12 inches off the pond bottom at the center of the pond.  The ends of the structure should be anchored to the pond sides just below the water line.  The center of the structure should be weighted to prevent floating.  Structure that is exposed above the water line will encourage filamentous algae growth. 

The following conditions should be present before stocking freshwater prawns into production ponds:

1.  Water temperatures between 65 and 95^o F.

2.  Water hardness or alkalinity 20 to 150 ppm.

3.  Water pH below 9.0.

4.  Continuous aeration with diffusers, paddlewheel, or axial flow pump.

5.  Dissolved oxygen above 3.0 ppm.

6.  Abundant food supply, zooplankton initially and a combination of pellet diet and natural food after the first month.

7.  Structure for molting prawns to escape cannibalistic males and as a surface for natural food organisms to grow on. (Structure is most important in ponds less than 1.0 acre in size.)

8.  Weed control by stocking grass carp.

9.  Predator control practices must be in force.

Zooplankton are the best food source for small prawns.  The pond should be conditioned with inorganic and organic fertilizer until a bloom of zooplankton and phytoplankton develops.  No prawns should be stocked if the pond water is clear.  Sample each pond for the presence of zooplankton, use a plankton net or compare water samples in clear glass containers.  Stock the small prawns when zooplankton numbers are greater than 925 per quart (1,000 per liter).  Accurate counts can be made using counting chambers or, compare water samples in small glass containers of known volume to estimate the zooplankton density.  Timing prawn stocking should be carefully managed so that the pond does not remain filled for more than one weed prior to stocking, five days is preferable.  Insects, frogs, and snakes can become established if the pond remains filled too long prior to prawn stocking.  A food supply will develop as the fertilizer stimulates phytoplankton and zooplankton growth.  Seeding the bloom with water from a nearby pond will hasten bloom development.  Prawns are expected to grow fast enough to be safe from all but the largest predators.

Clear water encourages aquatic weed growth.  Light penetration to the depths of the pond allows filamentous algae and rooted plants to grow.  Prawns do not really like light and will inhabit the deeper portions of the pond.  If weeds and filamentous algae become established, chemical treatment should be avoided.  Copper and most herbicides, including diquat, may be toxic to prawns at concentrations that are near or below the rates that effectively control weeds. Grass carp three inches long are stocked at the rate of 80 per acre when water is first added to the pond. Water soluble dyes can be utilized with some effectiveness for weed control.  Water dyes do not slow the growth of chara that has become establish in prawn ponds, perhaps because of the shallow depth or the ability of chara to grow under low light intensity.  If weeds are allowed to grow, prawn production will be lower than expected and harvest will be difficult.  Prawns will become entangled in weeds when the pond is lowered for harvest. 

Utilization of the dye, Aquashade^TM , or grass carp for weed and filamentous algae control has not been well documented for prawn culture.  Water soluble dyes may reduce the amount of primary productivity in ponds and therefore reduce prawn production by reducing the amount of natural food organisms.  Grass carp do not appear to eat the young prawns, so prawn survival does not seem to decrease when a generally herbivorous fish is stocked with prawns.  Culture systems using catfish, tilapia, or crawfish with prawns have been tested without losing the prawn population to fish.  However, in all cases of polyculture, prawn production was reduced from that obtainable in monculture competition or predation could contribute to lower prawn yields.  Polyculture does yield a higher yearly production when the weight of each species is totaled.

Pond preparation should emphasize management methods that develop a source of natural food for the prawns between the time of stocking and the first time complete feed is added to the pond.  During this period, ponds are managed to produce zooplankton that will serve as food for the young prawns.  Organic fertilizer added at the rate of 50 lb/A/wk can produce abundant zooplankton populations.  Cottonseed meal is a good source of organic material that is easy to handle, commonly available inGeorgia, and inexpensive.  Cottonseed meal has higher fiber than other sources of organic matter such as soybean meal, fish meal, or manure.  Cottonseed meal, alfalfa pellets, and distillers dried grains have been used successfully as prawn pond fertilizers. Inorganic fertilizers that have nitrogen and phosphorus should be added until a phytoplankton bloom develops in the water.  The visibility into the water column should be less than 18 inches.  Add 3 to 4 lb/A/wk of phosphorus in order to maintain a good bloom.  Common fertilizers include formulations of 8-8-8, 13-13-13, 0-45-0, 10-34-0, or 13-37-0.  The lime requirement for the pond should be considered before pond filling in order for a fertilization program to be effective.

Phytoplankton and zooplankton blooms can be started rapidly when water from a reservoir or adjacent pond is used to fill the prawn pond.  Addition of fertilizer to pond water that already contains zooplankton and phytoplankton causes rapid increases in numbers.  This practice should be encouraged in prawn culture in order to assure an abundant food supply for the young prawns and shade for the pond bottom to discourage aquatic weed growth.  It is important to note that all incoming water should pass through a screen that is small enough to retain fish eggs, small fish, and insects.  A filter fabric with 100 micron mesh will filter out all predatory organisms, however, a 300 micron mesh will suffice in most cases and require less maintenance during pond filling.  Ponds should be stocked within seven days after the pond is filled.  Predaceous insects will be at low densities during that time. 

The approximate pH maximum for freshwater prawn survival is 9.5.  Alkaline waters, obtained from aquifers in the southeast, have a pH of about 8.5.  There is little margin for pH increases that are likely to occur due to changes in the amount of carbon dioxide and dissolved oxygen in pond water.  Monitor pH daily, especially in the afternoon, to determine if high pH is present.  Gypsum (land plaster) has been added to maintain pH at about 8.3.  However, large quantities of gypsum may be needed.  Note that after a year or two of adding gypsum, sulfate accumulates in the bottom soils and may lead to production of poisonous hydrogen sulfide.  Using organic acids, such as vinegar, would be an option if an inexpensive source was available.  One practical method of pH control is the application of organic material to the pond at regular intervals.  Carbon dioxide production associated with decomposition of the organic material causes  lower pH measurements. Cottonseed meal or a mixture of cracked corn and soybean meal (1:2) have been used successfully to control pH.  A rate of 30 pounds of organic material per acre added three times each week can effectively limit pH to a range tolerated by prawns.  Organic matter decomposition produces carbon dioxide. 

Prawns should be stocked as early as possible with 15,000 to 40,000 juvenile prawns per acre.  These prawns will have completed 20 or more days in the nursery and have all of the anatomy of adult prawns.  Prawns are transported in fresh water supplied with oxygen from a gas cylinder and tempered into each pond over a period of about one hour.  Tempering is accomplished by adding pond water to the transport tank until the transport water is within one or two degrees of the pond water temperature.  Note that salinity changes between the nursery and the pond should be considered.  Periods of tempering must be extended if there is a difference between the salinity, hardness or alkalinity of the nursery and the pond water.

Size at stocking is important to prawn survival.  Differences in size between prawns usually result in cannibalism if the food supply is not adequate.  Even when food and fertilizer are added, larger prawns may attack smaller prawns when they are vulnerable after molting.  Grading prawns through a small bar grader may be necessary if the size variation is great.  Juvenile prawns should be stocked with others that are no greater than one quarter of an inch smaller or larger than each other.  Larger juveniles are usually are more suitable for larger ponds than small juveniles.  Prawns of 1.25 inch or larger will overcome predation from most aquatic insects that would prey on smaller prawns.  Stock prawns within five to seven days after beginning to fill the pond so that insects, frogs, and other predatory animals have less time to enter the pond.  If, for example, frogs spawn in a pond before the prawns are stocked, the pond should be drained and refilled to eliminate competition between the tadpoles and the prawns that will greatly decrease prawn production. 

Cotton seed meal or other organic material can be applied of 30 lb/A/day after prawns were stocked.  Inorganic fertilizer was added at the rate of 3 lb/A/week after prawns were stocked.  The fertilization schedule can be modified after 60 days or if zooplankton is monitored, when zooplankton counts reach a constant high count (Figure 3).

 

A 30% crude protein shrimp sinking pellet feed is applied starting on day 60 at the rate of 5% of the calculated weight of prawns.  Prawns can be sampled by seining between the structures or with lift nets.  Feeding rate can be reduced to 3% of the biomass when prawn average weight reaches 7 oz.  Carefully broadcast the feed over as much of the pond area as possible.

Table 1.  Freshwater Prawn Feeding Schedule.

Continuous aeration of prawn ponds has been a normal operating procedure.  The methods of aeration vary from blowers to paddlewheels.  Each method below is as if it were installed in a 1.0 acre pond that is 4 ft average depth.  A careful examination of the cost of each method will help guide your choice of aerator type and aeration schedule. 

Typically a 3/4 HP blower is attached to tubing with 0.5 inch slits every 18 inches (weighted diffuser tube) for a total of 116 ft of tubing length installed in an average water depth of 47 inches.  Another system uses a 1.5 HP blower attched by tubing to 3 inch medium pore silica diffusers which require 0.2 CFM/stone and 200 stones are needed when installed at 4 ft spacing. 

A 3/4 HP, 3,400 rpm motor with a single plastic propeller is floated in the pond in order to aerate and stir the pond water.  The aerator is more efficient than the diffusers, but may only aerate a localized area of the pond. 

This aerator consists of four plastic paddles powered by a 2 HP motor at 109 rpm.  The aerator is floated on the surface of the pond and paddles aerate and stir the water.  Large areas are aerated with this type of aerator, because a water circulation pattern is created. 

A motor (2 HP, 3,450 rpm) powers a propeller attached by a stainless steel shaft that directs water flow downward and aspirates air into the vortex.  A circulation is created below the water surface that moves the pond water away from the location of the aerator.

Table 2.  Comparison of performance and cost of aeration.

3.  Customer service and convenience.

4.  Customer entertainment.

5. Saleof associated items and services for added value.

Wholesale sales are lower priced and usually require processing the prawns and storage on ice or after freezing.  Each added step away from fresh product adds cost per unit of weight.  However, larger farms will utilize wholesale markets to sell the remainder of their crop after local sales have been satisfied.  Wholesale markets should be carefully identified before entering the freshwater shrimp enterprise.  Market planning should be the first step in your business plan.

Trout production can be accomplished in most parts of Georgia, during the winter months if the water temperature and dissolved oxygen are measured routinely and maintained in the range of tolerance of the rainbow trout.  Temperature optima are between 55 and 65 ^oF (13 to 18 ^oC) for rainbow trout.  Winter water temperatures, at Tifton, average about 61 ^oF (16 ^o) between November and March.  Winter water temperatures at locations north of Tifton are expected to be cooler, extending the growing season for trout.  The minimum temperature for rainbow trout growth is 38 ^oF (4 ^oC) and feed should be limited when water temperatures are between 38 and 55 ^oF (4 and 13 ^oC).

Stocking density of trout should not exceed 3,000 per acre.  A 1/6 to 1/4 pound trout (about 9 inches long) should be stocked in November.  Smaller trout do not take to feed as well as the larger trout.  After 4 to 4.5 months, the trout should be greater than 3/4 pound average weight.  Marketing should continue through the month of March.  Feed a good quality trout feed of 38 to 40 percent protein.  Do not exceed 30 pounds of feed per acre per day in order to preserve good water quality and to avoid the need for emergency aeration.

Dissolved oxygen concentration should be measured twice each day in order to plan aeration activity and guard against low dissolved oxygen.  A minimum of 4 ppm dissolved oxygen should be maintained.  At 1,000 trout per acre, aeration may not be needed until water begins to warm in March.

Harvest the trout with a seine or trap net before water temperatures exceed 65 degrees.  Water temperature should be taken in the morning and evening at approximately 18 inches deep.  The average temperature each day should be used to determine harvest timing.

Rainbow trout stockers may cost between $1.50 and $2.60 per pound.  Therefore a relatively large investment is made to stock an acre of water.  Survival of these large trout is expected to be very good if proper water quality is maintained and should average 95%.  Selling price should take into account that approximately $ 1.00/lb is needed to cover the cost of the stocker trout.  Trout feed may cost $400 to $500 per ton.  Feed conversion will be very good and 1.4 pounds of feed per pound of trout weight gain is to be expected.  Therefore $ .37/lb will be spent on feed costs to produce trout with an average weight of 3/4 pound.

Market price for rainbow trout ranges from $2.50 to $3.50 per pound when sold from the pond bank.  When 700 pounds of trout are produced per acre, net returns of $800 to $1,000 per acre are expected. 

The freshwater shrimp enterprise can be profitable when careful management is used along with local marketing.  Prawn production is hindered by a general lack of technical information, particularly the proper use of chemicals for water quality control and weed control. Variable juvenile size and quality, variable food supply, and pH control may be most important in determining prawn yields.  The addition of rainbow trout to the prawn rotation is viable and makes the enterprise more sustainable. 

D’Abramo, L.R., M.W. Brunson, W.H. Daniels, and M.E. Fondren.  1998.  Freshwater prawn hatchery and nursery management.  MS Univ. Ext. Ser. Pub. 2002.  9 pp.

Tidwell, J. H., S. Coyle, C. Weibel, and J. Evans.  1999. Effects and Interactions of Stocking Density and Added Substrate on Production and Population Structure of Freshwater Prawns,  Macrobrachium rosenbergii.  J. WORLD AQUACULTURE SOC.  30(2): 174-179.

Tidwell, J.H., S. Coyle, R.M. Durborow, S. Dasgupta, W.A. Wurts, F. Wynne, L.A. Bright, and A. van Aarnum. 2002. Prawn Manual.KentuckyStateUniversity,Frankfort,KY.

Wellborn, T. L.  1988a.  Site selection of levee-type fish production ponds.

SRAC Publication No. 100.  SouthernRegionalAquacultureCenter, USDA.  2 pp.

Wellborn, T. L.  1988b.  Construction of levee-type ponds for fish production.  SRAC Publication No. 101.  SouthernRegionalAquacultureCenter, USDA. 4 pp.

Wheaton, F. W.  1985.  Aquaculture Engineering.  Robert E. Krieger Publishing Company, Inc., Malabar, FL. pp. 414-462.