The hatchery and nursery stages are labor intensive and exacting, and require relatively high expertise for success. A limited number of postlarvae and juvenile suppliers currently exist, and an increase in demand will eventually lead to more enterprises that deal exclusively in the production and sale of seedstock.
A mature female produces approximately 28,571 eggs/oz (1,000 eggs/g) of wet weight. At the recommended range of holding temperature, a series of color changes (from bright yellow to orange to brown to a gray green) characterizes development of the eggs. Eggs with a gray-green color will hatch within 24 to 72 hours. Females with eggs in the advanced state of development can be removed from partially drained holding tanks and transferred directly to special hatching tanks containing water of similar temperature and a salinity of 0 to 5 ppt (g/L), where eggs usually hatch at night. By positioning a low-intensity light above the overflow pipe, larvae are attracted and thereby collected in a separate, adjoining tank. A small mesh screen, 3.5 x 10-5 to 4.7 x 10-5 in (90 to 120 micrometers), on the overflow pipe prevents larvae from escaping from the collection tank. Water from the collection tank then flows to another tank or back to the hatching tank.
During the following day, the concentration of larvae in the collection tank is determined and the appropriate number of larvae are then transferred to rearing tanks at an initial stocking density ranging from 189 to 300 per gallon (50 to 80/L). Stock the larvae collectively from eggs hatched during a 1- to 4-day interval. A following day's group of larvae should be stocked only after those stocked the previous day have been fed and evidence at least partially full guts. This procedure minimizes cannibalism of late-stocked individuals by earlier stocked individuals and ensures that a smaller range of larval stages occurs at any one time during the culture period. The duration of the harvest period is also minimized if a narrow range of larval stages (sizes) is maintained.
Clean, sterilize, and flush the larval culture system before initial filling. Water used for the initial filling should pass through a 5-micrometer bag filter. After the system is filled and operational, add a chlorine-based sterilizing agent to achieve a concentration of 10 ppt (g/L). Dechlorinating agents are not required if this sterilization procedure is performed several days before stocking. Such a protocol is recommended because the presence of dechlorinating agents has been implicated with mortality of prawn larvae. If only fresh water is available, you must add a commercially available salt mixture and thoroughly mix with the fresh water to achieve the appropriate salinity for culture. Use only proven high-quality salt mixtures because different salt mixtures can dramatically affect growth and cause mortality.
Water in the larval culture system is pumped from a collecting reservoir (sump) through a sand filter, passing an ultraviolet light unit and through a biological filter before it enters into the tank where the larvae are cultured. The volume of the biological filter should be approximately 6 percent of the volume of the entire culture system. The rate of water flow through the biological filter should range from 30 to 100 percent of the volume of the entire system per hour. Highest stocking rates of newly hatched larvae (100/L) will require the highest turnover rates (70 to 100 percent per hour). The sand filter should contain sand particles of an 850-micrometer size to achieve efficient removal of particulate matter before the water is again exposed to the ultraviolet light unit and the biological filter. The removal of particulate matter from the water enhances the efficiencies of the ultraviolet light and biological filter. The ultraviolet light exposure dramatically reduces the concentration of bacteria and accordingly reduces the potential incidence of pathogenic bacteria. The sand filter must be flushed (backwashed) -- once to several times daily, depending upon the size of the larvae and the amount of food fed -- to avoid accumulation of particulate organic material, which can clog or cause channeling, thereby reducing the efficiency of removal. Other types of systems designed for the removal of particulate material from recirculating systems are available.
The biological filter media are activated in a separate preconditioning container by introducing other media that already have established populations of nitrifying bacteria. Once appropriately conditioned, quantities of the biofilter media are then transferred to the actual biological filter unit as needed (i.e., as the biomass of the larvae in the culture tank increases). Temperature, 82.4 to 86.0 °F (28 to 30 °C), and salinity, 12 ppt (g/L), in the culture and activating tanks should be the same; constant, vigorous aeration is required. The procedure for activating substrate for the biological filter follows:
One procedure includes:
Hatching of cysts is best achieved in conical bottomed, funnel-shaped PVC containers that are equipped with a valve at the narrow end to facilitate separation and removal of nauplii and wastes. Stock cysts at <= 0.20 oz/gal (1.5 g/L) in natural or artificial salt solutions at a salinity of 10-12 ppt (g/L). The hatching medium can be enriched with 2 ppt (g/L) of sodium bicarbonate (NaHCO3). The pH should not drop below 8, and temperature should be kept within the range of 77 °F to 86 °F (25 to 30 °C). Provide aeration to maintain dissolved oxygen levels above 2 ppm (mg/L). The hatching tanks should be well illuminated from above, with four 60-watt fluorescent lightbulbs (1,000 lux) at a distance of 7.87 in (20 cm). After approximately 24 hours, harvest hatched Artemia nauplii according to the following procedure:
The hatching rate of cysts varies according to storage time and conditions as well as origin and commercial brand. Generally, 150,000 hatched Artemia nauplii can be expected from 0.035 oz (1 g) of cysts. Most larvae begin feeding one day after hatching (larval stage 2). Frequent feedings of live food, Artemia nauplii, rather than one or two feedings spread over a long period of time, should be practiced because nutritional value of Artemia in the water column will decrease over time as the Artemia remove the nutrients contained in the yolk sac.
Generally, the initial feeding of the prawn larvae consists of frozen cubes of Artemia nauplii followed by a 1.1 qt (1 L) volume of newly hatched live Artemia that have passed through a 0.0047-in (120 micrometer) harvest screen. The Artemia nauplii to larva ratio should be initially maintained at 6-8:1, and the density of Artemia should be checked at 20-minute intervals. When the ratio falls below the recommended level, add additional Artemia in .55-to 1.1-qt (0.5 to 1 L) increments during the early part of the larval cycle and at 1.1-to 1.7-qt (1 to 1.5 L) increments in the later stages of the cycle. A suggested feeding rate of nauplii, according to day poststocking and stage, is presented in Table 1.
No later than midmorning, collect a sample of larvae (50-100), and examine them under a dissecting microscope to determine whether their guts are full. Full or mostly filled guts indicate healthy individuals. Empty or almost empty guts are an indicator of poor culture conditions (i.e., poor water quality, high levels of bacteria, or insufficient levels of food provided). Excess Artemia that are produced should be frozen in ice cube trays to be available for use during early morning or when poor hatches occur.
|Day of cycle||Stage index||Artemia per larva||Supplemental feed||Particle size||Flushing screen|
|a.m.||p.m.||upper (mg)||lower (mg)||(micrometer)||(micrometer)|
Ingredient composition of supplemental diet
Percent wet weight
Cod liver oil
Beef liver powder
After the transfer has been completed, pump 1/2 to 2/3 of the water from the tank where the postlarvae remain to another holding tank and sterilize for future use. The postlarvae are now ready for acclimation to fresh water, which should be added gradually so the salinity eventually decreases to 0 ppt within a 24- to 36-hour period. At this time, determine the mean weight of individual postlarvae by weighing a specific number of postlarvae. In order to estimate the total number of postlarvae produced per production cycle and to control the density stocked into tanks in the nursery phase, weigh the groups of postlarvae collected as they are transferred to the nursery. Knowledge of the total biomass (weight) harvested and the mean individual weight will permit an estimate of numbers stocked. Generally, survival in the hatchery culture phase ranges from 40 to 80 percent.
Nursery culture can be accomplished in a variety of ways, including small enclosed ponds or tanks in climate-controlled buildings. To conserve water and heat, water recirculation systems are recommended, but you can also use flow-through systems equipped with heaters. The depth of the ponds or tanks (pools) should not exceed 4 ft (1.2 m) and should be equipped with structure (artificial habitat) throughout the water column to increase the total available surface area. The habitat will result in wider distribution of the prawns, significantly reducing the incidence of cannibalism. To achieve the best growth and survival, an initial stocking density of not greater than 19-23 PL/gal (5 to 6 postlarvae/L) is recommended, and water temperature should range between 78.8 to 82.4 °F (25 to 28 °C). Postlarvae may be fed a commercially available trout diet containing a high level of crude protein and energy and being a particle size that can be readily consumed. The total daily ration is divided into two or more separate feedings. Three times per week frozen beef liver is fed as a substitute (on a dry weight basis) for one of the trout diet feedings. The level of the daily ration may need to be adjusted, based upon whether or not the amount of food provided is entirely consumed. Under these culture conditions, the nursery phase should produce within 50 to 60 days individuals with a mean individual weight of 0.011 oz (0.3 g). The nursery phase should not exceed 60 days due to the increased incidence of mortality by cannibalism as the individual mean weight increases, and the increased potential for the occurrence of adverse conditions of water quality. Generally, 65 to 75 percent survival can be expected at the end of the nursery phase.
Size grading can be performed with modified bar graders or with those that are used to grade fish. The type of separation achieved will depend upon bar width as well as the weight (size) distribution of the population of prawns. Experiments have demonstrated that a good relationship exists between bar width and mean weight of the largest prawns that pass through oriented parallel to the bars. Determine the prawn size (weight) -- bar width relationship for the specific size grading technique used. A 50%-50% (upper-lower) or 40%-60% (upper-lower) numerical separation is advised so the entire population can be used for stocking. However, even both populations arising from a 70%-30% (upper-lower) separation have been successfully used. Conduct size grading with sufficient aeration to avoid stressful conditions. Juveniles move toward a flow of water, and this behavior may assist in the development of passive grading techniques. Other, more active, grading techniques would involve the movement of a grader through a population or the movement of a population through a stationary grader. No specific grading procedure is recommended. The choice would be based upon the experience and resources available to the culturist.
By Dr. Louis R. D'Abramo, Professor, Dr. Martin W. Brunson, Extension Leader/Fisheries Specialist, and Dr. William H. Daniels, former Research Assistant, all with the Department of Wildlife and Fisheries, and Mack E. Fondren, Research Assistant, Animal Research Center.
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