Hatchery Systems
Greenwater System of Freshwater Prawn Culture
A more common alternative to the ‘clearwater' system for flow-through hatcheries is known as the ‘greenwater' system. In the greenwater system, a mixed phytoplankton culture in which Chlorella spp. is dominant is maintained in separate tanks. Its cell density is about 750000-1500000 cells/ml. A fertilizer solution in tap water is added to the tanks at least once per week to maintain the culture. This solution provides a mixture of 4 parts of urea to 1 part of NPK (15:15:15) garden fertilizer, applied at the rate of 185 g per 10 m3 tanks. Tilapia (Oreochromis mossambicus) is held in the tanks at the rate of about 1 per 400 l to graze on and control filamentous algae. Copper sulphate, at the rate of 0.6 ppm is added to the greenwater tanks once per week to control rotifers. The tilapia also helped to fertilize the culture. The sodium salt of EDTA (ethylene diaminetetraacetic acid) is included in the greenwater culture at 10 ppm as a chelator. The greenwater is prepared at the same salinity as the larval rearing water. Greenwater does not thrive at more than 12 ppt salinity. The greenwater culture is never used for larvae if the culture is more than three days old. Part had to be discarded or used for filling larval tanks and the rest diluted regularly to avoid phytoplankton ‘crashes' occurring in the larval tanks. Although the greenwater system may have some advantages, it is difficult to manage successfully and adds more complications to the hatchery process. For this reason, most commercial freshwater prawn hatcheries now use clearwater systems of management, whether they are flow-through or recirculation.
Semi-Closed, Two Phase Clear Water Larval Rearing Technology
Central Institute of Freshwater Aquaculture has developed and standardized a two-phase clear water technology for larval rearing that is suitable for non-coastal hatcheries. This technology can be suitably modified to suite other locations also. Healthy mother prawns (bearing grey eggs on their pleopods >50 g) are selected from the broodstock pond/tank and disinfected with 0.3 ppm copper sulphate or 30 ppm formalin for 30 min. Mother prawns are then stocked @ 100-150 g/m2 (2-3 nos of ~ 50 g female) in brackishwater (salinity of 5‰) and reared till hatching. Tanks are checked daily for appearance of larvae.
Once hatching occurs it may continue for 24-48 h. The spent female is removed from the tank and released back to the broodstock pond. The salinity of the larval rearing medium is then increased to 12‰ and the rearing is continued in the same tank. In the first phase the larvae (Zoea I) are stocked in cylindro-conical tanks at a high density (200-300 larvae/l). About 50% of the medium is usually exchanged every other day with fresh medium of identical salinity. The larvae are reared for about 10-12 days in this phase. In the second phase, the advanced larvae are stocked in larger tanks with a greater surface area at the rate of 50-80 per litre and reared till metamorphosis. About 50% of the medium is exchanged every alternate day.
The freshly hatched Artemia nauplii are given as live food to the prawn larvae, 4-5 times per day in the early stages (Stages II to V or VI) and later, once during late evening in combination with wet larval feed which is usually given during day time. The brine shrimp nauplii are fed to the prawn larvae at the rate of 5 to 50 nauplii per larva per day. About 2 kg of Artemia cysts are required to produce one-lakh post-larvae.
Wet larval feed (egg custard, minced fish/mollusc flesh; protein level> 50%) is fed @ 50-200 mg/larva/day depending on the larval stage. The wet feed is given from 8 am till 2 pm at one-hour interval. The larval rearing tanks are cleaned daily by siphoning off excess food particles and metabolic waste from the bottom of the tank. This is done after stopping aeration, preferably in the evening hours before exchange of water and introduction of live food (Artemia nauplii). Daily monitoring of temperature, salinity, pH and dissolved oxygen levels is essential to maintain the water quality at optimum levels. The optimum ranges of water quality parameters for successful seed production are given below.
Water temperature - 29-31ºC
Salinity - 10-13‰
pH - 7.0-8.5
Dissolved oxygen - >5 ppm
Nitrite - <0.1 ppm
Ammonia (NH3 -N) - <0.1 ppm
The appearance of first post-larva is usually observed 20 days after hatching, normally between 22 and 26 days (at 28-32°C) and 90% larvae metamorphose within next 10 days. The seed production normally ranges between 35-40 per litre and the cycle lasts for 35-40 days. The post-larvae are gradually acclimatized to the freshwater and reared at high densities (2000-5000/m2 ) for 10-15 days in hatchery. The post-larvae are fed with formulated diet @ 100% of the biomass per day. After a week post-larvae are suitable for stocking in grow-out ponds.
Harvest of post-larvae
New postlarvae (PL) are about 7-8 mm long. Although PL can withstand the physiological shock of sudden transfer from 12 ppt water into freshwater, it is not recommended to harvest them from the larval tanks and transfer them directly into holding tanks containing freshwater. The animals are best acclimatized to freshwater in the larval tank. Once the majority of larvae have metamorphosed (at least by day 32-35) water level in reduced in flow-through system tanks to about 35 cm. The PL can then be harvested and transferred, or the larval tanks refilled to 70 cm with freshwater and the animals temporarily held in them. If the latter is done, the PL should only remain in the larval tanks for a few more days, with frequent water exchange, before transfer to a larger holding tank.
The best way to harvest PL from the larval tanks is to reduce the water level and then remove them in dip nets. Most flow-through hatchery operators harvest their post larvae only once, at the end of the production cycle.
