FRESHWATER PRAWN PRODUCTION IN GEORGIA

SUMMARY

Freshwater prawn are being cultured on an experimental basis by 20 or more producers in Georgia. Several producers are also distributing juvenile prawns. At least two hatcheries are being developed in the state in order to provide a local source of seedstock. Experience with culture of the prawn results in varied returns from very low to approximately 1,500 lb/A/yr. Successful efforts must manage the water quality during the first days after stocking in order to provide adequate food for the young prawns. Growing seasons in Georgia range from 120 to 180 d.

INTRODUCTION

A number of producers attempted to raise the freshwater prawn, Machrobrachium rosenbergii, in Georgia in 1999 and 2000. These producers installed nursery systems and ponds to grow the prawns for the food sized market. As can be expected with new endeavors, some successes as well as failures occurred as these producers used various adaptations of recommended methods for prawn production.

The basis system for prawn production involves three phases: the hatchery phase, the nursery phase, and the grow-out phase. Mature prawns from the grow-out phase are then uses as spawners for the hatchery phase. Hatcheries have developed in Texas, Mississippi, Kentucky and Hawaii. 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 30 to 60 d 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 3 to 5 mo depending on the size of prawn desired. Ponds are stocked with 16,000 to 25,000 juveniles per acre in order to obtain 600 to 1,500 pounds of prawn by the end of the growing season.

In order to examine the application of prawn production techniques developed at cooler and more tropical locations, a prawn production trial was conducted in Tifton.

METHODS

The prawn production systems were managed using the following assumptions in order to meet the biological requirements of fresh water prawns.

1. Water temperatures between 65 and 95^o F.
2. Water hardness or alkalinity near 100 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 climb out of reach of cannibalistic males and as a surface for natural food organisms to grow on.
8. Weed control.
9. Predator control.
   

The juvenile prawns were obtained from a nursery in Georgia and came originally from Texas brood stock. Sizes ranged from 0.25 to 1.0 in at the time of stocking. Four 0.1 A ponds were stocked at the rate of 16,000 prawns/A. Zooplankton populations were measured using a zooplankton net at weekly intervals to estimate the availability of food organisms. Prawns were harvested in August and October by seining and finally by draining of ponds.

RESULTS AND DISCUSSION

Prawn production is increased by obtaining juveniles of nearly the same size. Some nurseries have better systems than others. If widely different sized juveniles are obtained, survival in the early days after stocking into ponds may be low. Larger prawns will eat the smaller ones. Smaller prawns may be nutritionally challenged and less able to survive. It is difficult to count juveniles for stocking due to their small size. If large and small juveniles are present at stocking, grade them before an estimate of average weight is made. Separate the size groups into different ponds. Variability of more than 0.5 in between the large and small juveniles in this trial caused low survival due to cannibalism

Zooplankton provide 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. Timing prawn stocking should be carefully managed so that the pond does not remain filled for more than two weeks prior to stocking, one week is preferable. Insects, frogs, and snakes can become established if the pond remains filled too long. A food supply will develop as the fertilizer stimulates phytoplankton and zooplankton growth. Seeding the bloom with water from a nearby pond will hasten the development of a bloom. With an adequate food supply, the 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 may be toxic to prawns at concentrations that are near or below the rates that effectively control weeds. Water soluble dyes can be utilized with some effectiveness for weed control. Grass carp can be stocked early in the season before any weeds are present but the effect of grass carp on prawn production is not known. If weeds are allowed to grow, prawn production will be lower than expected and harvest will be difficult. Prawns will become entangled in the weeds when the pond is lowered for harvest. Prawns were killed by both copper sulfate and diquat treatments in this trial. Water dyes did not slow the growth of chara in any of the four ponds.

Utilization of aquashade or grass carp for weed and filamentous algae control has not been well documented for prawn culture. Aquashade and other 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 may eat some of the young prawns so that prawn survival may be less when the generally herbivorous fish is stocked with prawns. Culture systems using catfish and prawns or tilapia and prawns have been proposed. In all cases of polyculture, the prawn production is reduced from that obtainable in monculture. Polyculture does yield a higher over all yearly production when the weight of each species is totaled.

Extension service fact sheets from Mississippi and Kentucky with information about prawn production did not emphasize the management methods to use in the time between stocking juveniles and the time that feed is first offered to the prawns. However, during this period, ponds are managed to produce zooplankton that will serve as the food for the young prawns. Organic fertilizer added at the rate of about 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 in Georgia, and inexpensive. Cottonseed meal has a higher fiber content than other sources of organic matter such as soybean meal, fish meal, or manure and breaks down more slowly, so negative water quality effects are less common when it is used. 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 in in a properly fertilized pond. 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 must have been considered before the pond was filled with water 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 relatively large quantities of microorganisms 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. 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 unwanted predators, however, a 300 micron mesh will suffice in most cases and requires less maintenance. Ponds should be stocked with the prawns within 7 d after the pond is filled. Predaceous insects will be at low densities during that time.

Feeds and feeding gradually change from a fertilization schedule to feeding a slow sink pellet. Common agricultural byproducts such as cottonseed meal and distillers grains may be utilized to feed the prawns during the first two mo of grow-out. However, during the last 30 to 60 d of production, a pellet must be utilized. Scatter the feed as evenly over the pond bottom as possible. Narrow pond designs are common in prawn culture in order to allow easy feed application. Feeding rate tables have been proposed, however, sample the prawns periodically to determine an average weight, then calculate the proper feed allotment based on feeding 4% per d of the total prawn weight. Prawns use natural food to supplement the diet provided, so a very complete diet must be provided in plastic lined ponds or tanks.

Harvesting prawns can be similar to seining other aquatic animals if the pond is free of obstructions. Yields from this trial ranged from a few prawns to the equivalent of 700 lb/A. Variability among the four ponds was similar to that reported by prawn producers in Georgia between 1999 and 2000. A seine with ½ in mesh was utilized for harvest. Once the prawns were collected, the large prawns were sorted from the small ones with a bar grader and by hand picking. Small prawns (less than 0.1 lb) were returned to the pond for 30 d or more of additional growth. Removal of the large blue claw prawns allows other prawns to grow to a larger size. Prawns require oxygenated water to survive, so plenty of aeration should be utilized when harvesting. Provide a substrate in the holding tanks if you want to keep the harvested prawns alive. The substrate will allow prawns to seek refuge from their aggressive companions. Prawns jump considerable distances so that a cover should be placed over the holding tank immediately after it is filled. Although prawns can walk on land, they seldom survive on dry land for more than a few minutes. To preserve the best appearance of the large blue claw males, close their claws with a small rubber band prior to holding in tanks. Cool water temperatures, 68 to 72 degrees Fahrenheit, slow the prawns down so that they are less aggressive.

The final harvest of prawns should occur before pond water temperatures fall below 60 degrees Fahrenheit. The giant prawn is a tropical to sub-tropical animal and has slow growth at low temperatures. Little or no growth is expected when water temperature is less than 70 degrees Fahrenheit. In Georgia, the growing season may end in September to October. A growing season of 180 d may occur in the southern third of the state while only 120 d may be safe in the northern third. Indoor culture of the prawn is difficult and requires a great investment in tanks, filters, and electricity. Only the hatchery and nursery phases are practical for indoor culture of prawns.

CONCLUSION

Prawn production requires careful management and is hindered by a general lack of technical information, particularly the proper use of chemicals for water quality control and weed control. The evaluation of prawn production in Georgia will continue for one more season. Variable juvenile size and quality, variable food supply, and pH control may be most important in determining prawn yields.