How to raise Freshwater prawns
Freshwater prawns, like all crustaceans have a hard outer skeleton or shell that must be shed regularly in order for growth to occur. The process of shedding the shell is called "molting," and weight and size increases occur principally soon after each molt. Because of these periodic molts, growth occurs in distinct increments, rather than continuously.
Older juveniles and adults usually have a distinctive blue-green color, although sometimes they may take on a brownish hue. Color is usually the result of the quality and type of diet. Adult males are larger than the females, and the sexes are easily distinguishable. The second walking legs or claws (chela) and the head region of males are larger than those of the females.
The base of the fifth or last pair of walking legs (periopods) of males is expanded inward to form a flap or clear "bubble" that covers the opening (gonopore) through which sperm are released. The walking legs of males are set close together in nearly parallel lines, with little open space between them, which helps distinguish immature males from females. A wide gap exists between the last pair of walking legs in females, and they have a genital opening on the base of the third pair of walking legs.
Three types of males have been identified, based upon external characteristics. Blue-claw (BC) males are easily distinguishable and are characterized by long, spiny blue claws. Two other classes of non-blue-claw males exist, orange claw (OC) and strong orange claw (SOC) males. The transformational sequence is from OC to SOC to BC males. Smaller OC males (< 10g) grow slowly but are more reproductively mature than are other OC males.
BC and some smaller OC males are the most reproductively active and successful at mating. The BC maintains a territory associated with a group of females that are ready for mating, and protects them during the vulnerable period just before and after molting. Small OC males eventually grow and transform to SOC (strong-orange claw) males before becoming BC males. BC males undergo an extended period of nonmolting. As the BC male ages, reproductive capacity is lost. Eventually, the BC male molts and returns to a growth phase. Later, its reproductive capacity is regained.
Brood stock collection
Collect females with brown or gray eggs from your brood stock holding tanks. Avoid choosing females with orange or yellow eggs because it will be several days before they will hatch. Handle the berried females carefully so they don’t loose their eggs. Place females with brown or gray eggs into the holding tank of the hatching system.
Females generally become reproductively mature within 6 months. Mating can occur only between hard-shelled males and soft-shelled females, i.e., females who have just completed a premating or prenuptial molt. The male deposits sperm into a gelatinous mass that is held underneath the body of the female, between her fourth pair of walking legs. Eggs are laid within a few hours after mating and are fertilized by the sperm contained in the gelatinous mass attached to the outside of the female's body. The female then transfers the fertilized eggs to the underside of the abdominal (tail) region, in a "brood chamber," where they are kept aerated and cleaned by movement of the abdominal swimming appendages. Eggs remain attached to the abdomen until they hatch. The number of eggs produced at each spawn is directly proportional to the size of the female. As long as water temperature exceeds 70 °F, multiple spawns per female can occur annually. Females carrying eggs are termed "berried females." The bright-orange color of newly spawned eggs gradually changes to orange, then brown, and finally gray about 2 to 3 days before hatching. At a temperature of 82 °F, the eggs hatch approximately 20 to 21 days after spawning. Newly hatched freshwater prawns enter into a larval phase of growth and metamorphosis.
Hatching and Stocking
Place females with brown or gray eggs into a small hatchery containing freshwater. Add salt over 6 hours until salinity is 12 ppt. Maintain water at 28-30C and provide aeration until hatch. Larvae are positively phototactic, i.e. they will congregate near a light source. Hatched larvae are easily captured in a collection tank by attracting them towards a light source. Siphon larvae into a bucket and transfer to hatchery culture tank. Maintain water at 28-32C and a salinity of 12ppt with sufficient aeration for the next 28 days.
After hatching, larvae are released and swim upside down and tail first. The larvae cannot survive in freshwater beyond approximately 48 hours and migrate to brackish water with a salinity of 10 to 14 parts per thousand (ppt). Larvae are aggressive sight feeders and feed almost continuously, primarily on small zooplankton, worms, and larval stages of other aquatic invertebrates. Larvae undergo 11 molts, each representing a different stage of metamorphosis. Following the last molt, larvae transform into postlarvae. Transformation from newly hatched larvae to postlarvae requires 15 to 40 days, depending upon food quantity and quality, temperature, and a variety of other water quality variables.
After metamorphosis to postlarvae, the prawns resemble miniature adult prawns, about 7 to 10 mm (0.3-0.4 inch) long and weighing 6 to 9 mg (50,000 to 76,000 per pound). The prawns behaviorally change from living suspended in the water column to principally bottom dwelling, crawling individuals. When they do swim, they move like adults with the dorsal (back) side uppermost and in a head-forward direction.
Postlarvae can tolerate a range of salinities and migrate to freshwater upon transformation. In addition to the food they ate as larvae, larger pieces of animal and plant materials will be ingested. The diet includes larval and adult insects, algae, mollusks, worms, fish, and feces of fish and other animals. At high densities, or under conditions of food limitations, prawns become cannibalistic.
Postlarvae are translucent and may have a light-orange-pink head. As they change to the juvenile stage, they take on the bluish to brownish color of the adult stage. Juveniles are intermediate in size between postlarvae and adults; however, no standard definition for the juvenile stage exists.
The nursery stage is the easiest step of culture. In temperate climates, the requirements to nurse prawn are large tanks containing heated freshwater that can be constantly exchanged or filtered to maintain ideal water quality. The tanks must contain substrate such as netting material. This will provide added habitat for the postlarval prawn. The prawn will grow from approximately 0.01g to 1g during this 30 day period. The pictures below show a nursery tank and the size of the prawn during this stage of culture.
Substrate should be provided in the nursery tanks. Provide substrate in the tank at six to twelve inch intervals. The ideal amount of substrate to use has not been identified experimentally but the amount provided is directly related to the number of prawn that are alive at the end of the nursery stage of culture. Substrate should stay off the bottom of the tank and can be hung either horizontally or vertically. We use bird netting or plastic warning barrier fencing for substrate.
Artemia culture and feeding practices
Fill Artemia hatching tank with 12 ppt saline water. Add Artemia cysts at 1 g/L. Incubate at 28C for approximately 24 hours. Collect Artemia in a nitex screen and separate hatched nauplii from unhatched cysts.
Feed collected Artemia to larval prawn. Maintain ideal water quality and feed larvae 6 times per day to satiation. Feed for 28 days from sunrise to sunset (6 X/day). Clean tanks twice daily to remove excess feed and waste. When larvae metamorphose to post-larvae, begin adding freshwater slowly and stock post-larvae into nursery tanks.
Feeding rates are calculated by estimating the biomass of all postlarvae stocked and subtracting an assumed biomass loss caused by mortality. A sample of postlarvae should be taken each week to estimate the biomass in the tank. Feed postlarvae 15% of their biomass the first week, and decrease one percent a week until the nursery stage is complete. Therefore, you have an increasing biomass because the postlarvae are growing and you feed them one percent less each week because their feed requirements decrease as they grow. Remember you must maintain ideal water quality so don't overfeed.
Counting a large number of prawn to stock into the ponds is not practical. Therefore, count several samples of 100 prawn and place them in a small net and spin the net to remove water. Calculate the mean weight of one prawn and multiply by the number of prawn you plan to stock. Collect weights of all prawn you stock. This will provide a reliable estimate of the number of prawn stocked. It does not hurt the prawn to be spun dry for a few seconds.
Growout of prawns to market size (30-100g) is best accomplished by stocking them into ponds. The average stocking density is 20,000 per acre and you can expect 500-900 pounds per acre at harvest. Harvests greater than 1200 pounds per acre have been accomplished in research ponds (0.25 acres) but have never been reported from commercial size ponds. The average price for the prawns is \$4.50 per pound ($3.25-$8.00) for whole prawns and \$8.00 per pound ($6.00-$16.00) for tails only. Most producers sell the prawns themselves but there is a developing industry which includes processing and wholesale distribution of the product.
Ponds should be designed to completely drain in less than a day. To accomplish this there must be a slope of approximately 4 inches per 100 foot of pond length. There must be one or two drain pipes in the deepest part of the pond depending on the size of the pond. Approximately a 10 inch drain is needed for each acre of water (i.e. a 3 acre pond needs two15 inch drains). A catch basket should be constructed on the outside of the levee to collect any prawns which exit through the drain. This method of harvesting is very efficient if the pond drains rapidly with low volumes of water remaining. The prawns will follow the flow of water and can be easily collected in a catch basket on the outside of the levee. The remaining prawns will be concentrated in the catch basin in the pond and can be removed with a net and boom truck. It is inevitable that some prawns will have to be collected by hand from the bottom of the pond. Keep the bottom of the pond smooth without any holes or depressions which will retain water and prawns as the pond drains. With proper pond design, harvesting can be time and labor efficient.
Substrate can be used in ponds to provide safe habitat for prawns to molt. Production increases when substrate is added to ponds. Several studies are currently being conducted to identify the optimal stocking and substrate densities for increasing pond production.
Prawn cannot be completely harvested from a pond with a seine. However, some farmers like to partially harvest a pond before drain harvesting. This increases the labor involved in harvesting but allows for processing and marketing of the prawns over a few days. There are two efficient ways to completely harvest a pond. One method is to drain the pond and remove prawns by bulk from the deep part of the pond or in a catch basin. Another way is to collect prawns on the outside of the levee in a catch basket. This can be accomplished by sloping the bottom of the pond so it drops at least four inches per 100 feet. This concentrates the prawn in the deepest part of the pond when the pond is drained and makes it easier to remove the prawns. The construction of a catch basin is also effective if the pond bottom is sloped enough to concentrate the prawns in the basin.
Prawns are then unloaded, chilled, and sold whole to local markets or the heads are removed and the tails are block frozen for distribution to other markets.
If the pond is properly managed, a yield of 500-800 pounds per acre of 8 to12 count prawns can be achieved. A prawn with an average weight of 45g is a 10 count prawn, or simply 10 prawn weigh one pound.
There are three phases of culture of the freshwater prawn -- hatchery, nursery, and pond grow out. If you are contemplating starting a freshwater shrimp production enterprise, forego, initially at least, the hatchery phase and possibly the nursery phase by purchasing juveniles from a supplier. As production increases and you are successful at pond grow out of the animals, you should begin plans to develop a nursery, and possibly a hatchery. There are a limited number of juvenile prawn suppliers, but increased demands will eventually lead to a need for more enterprises that deal exclusively in the production and sale of seed stock.
Freshwater Prawn Sources
Freshwater Shrimp/Prawn Macrobrachium rosenbergii, Juvenile Domestic Suppliers: