Freshwater prawn

Collection of freshwater prawn

The prawn are collected by nets from ponds, rivers and transferred to the nicely aerated tanks. The transport of these collected prawns is done in oxygenated plastic bags or bamboo baskets with aquatic plants.

Maintenance of brood stock

For developing a brood stock early juveniles are stocked in the ponds at a rate of 3-4\m²

and adults @ 2 nos \m². The sex ratio can be maintaining 1:4 (male: female). The prawn can be fed on a pelleted feed containing 35-40% protein which can be prepared from groundnut oil cake(60%) fish meal (20%)deoiled rice bran (18%)and vitamin and mineral mix(2%). They are fed 3-4% of the total body weight about 3 times per day. Prawns attain maturity within 4 to 7 months depending on feed, temperature and other ecological conditions.

Broodstock and spawning tank management

To get enough quality eggs, careful management of the broodstock is required. The depth of the water in holding tanks should be 0.9-1.0 m. Daily, in the morning and evening,. 50 per cent of the water should be changed. Enough shelter should be provided at the bottom of the tank to reduce stress. Tank covers, or a shed, should be installed for shade. Rainwater stimulates sexual activity of the prawn, so better results can be expected if rainwater is allowed to fall into the holding broodstock tank. Feed equivalent to 5 per cent of the bodyweight of the stocked prawns should be supplied twice every day, in the morning and evening. It is preferable to supply raw food in the night and prepared food during daytime. Chopped fish and mussel, small shrimp, chopped fish, and adult Artemia are good for broodstock. Dry pellets with good food value can be provided, if available. Before the food is given everyday, the bottom of the tank should be cleaned by siphoning. This removes uneaten food, leftover moults and faecal waste. The tank bottom and wall should be brushed and cleaned every two days. The tank and broodstock can be disinfected by introducing 20 ppm formalin solution in the tank, followed by a 100 per cent change of water after 24 hrs. This must be done very carefully. From time to time, the water level should be decreased and injured prawns removed. Berried females should be transferred to the spawning tank as they become available.

Broodstock collection and maintenance

The female prawn carries the fertilized eggs under her abdomen in a brood chamber. Prawns that are carrying eggs are referred to as ‘berried’. They are generally found in rivers and their tributaries, canals, ponds and deep depressions. Berried prawns can also be produced in the holding tanks in a hatchery. During cool periods, broodstock can be produce Dina hatchery under controlled temperature conditions. Berried female prawns are available throughout the year in the lowlands of Malaysia, Thailand, Indonesia, southern Vietnam and similar locations where water temperatures remain high through the year. In Bangladesh and West Bengal, natural reproduction occurs only during the warm months — from late March through September. Transport of berried prawns is similar to that of adult prawn, but because of the eggs in the abdomen, they need to be handled with more care than immature prawns. Berried female prawns should be held in individual perforated plastic tubes if they are to be transported long distances. The ends of the tube are closed with gauze held in place by rubber bands. The tubes are transported in plastic bags containing oxygenated water. After broodstock collected, they should be disinfected with formalin. Weak, wounded and diseased animals should be discarded and only healthy and disease-free berried prawns stocked or transferred to the hatching tank. In the hatching tank, it is advisable to stock four prawns/sq.min water of depth 30-40 cm. The tank bottom should be cleaned every morning and evening and 50 per cent of the water exchanged. The salinity should gradually be increased to 12 ppt. The feed ratio should be 5 per cent of the total bodyweight of broodstock. Bivalve meat, snails and worms can be fed. Aeration should be continuous and the temperature maintained at 28-29°C.If temperature is not controlled, the growth of embryo will be delayed and the newly-hatched larvae will be weak and undersized. The main purpose of this tank is to provide proper conditions for the embryo to grow. Full development of the eggs in the abdomen of the female prawn takes about 19 days. The female prawn remains busy during these 19 days by brooding the eggs. As the eggs develop their bright orange colour changes to grayish colour.

Selection and disinfection

Egg development should be observed every alternate day a’ conical scoop net is used to take out each prawn and its yellowish ventral egg sac examined. When the colour of the eggs in the sac becomes dark grey, the prawn should be transferred to hatching tanks after disinfection. Disinfection is done by keeping them in aerated water with 25 ppm formalin for 1½hour. When the elliptical eggs are ready to hatch, fully developed larvae may be seen inside the egg with the aid of a microscope. Berried females should be selected carefully, applying the following criteria. They should be:

• Healthy and disease free;
• Strong and active;
• Bright-coloured;
• Laden with a large number of eggs;
• As large as possible.

The chances of larvae surviving from prawn’s havingthese qualities are good. Their growth will also be fast.

Hatching tank management

Hatching can be done in the larvae rearing tanks. However, the use of a separate hatching tank helps to prevent the spread of disease to larvae rearing tanks. Larvae rearing density is also more easily controlled. In case separate tanks are used, for every 100 g prawn weight a tank or aquarium of at least 500 1 capacity is required. Before stocking prawns in a hatching tank, it should be filled to a depth of 30-40 cm with treated 12 ppt brackish water. The sodium salt of ethylene diamine tetracetic acid (EDTA) appears to have very beneficial effects on the hatching rate and larvae survival, so 5-10 ppm EDTA should be added to the water in the hatching tank. Aeration should be continuous and temperature maintained at 30°C.After hatching of larvae, spent broodstock should be returned to the holding tank or broodstock pond. No feed need be supplied to berried prawn in the hatching tank. During the time they spend in this tank, they are busy taking care of their eggs and do not feed. Larvae generally hatch during the first half of the night, although sometimes hatching may take place during late evening. In some cases, partial hatching also occurs. Movement of larvae starts five minutes after hatching. At this time, larvae swim in a head-down position. Larvae are made to concentrate in one corner of the hatching tank by covering all but a small portion of the surface with an opaque material. Larvae are attracted to the light and will gather in the exposed corner, from which they can be easily removed with a siphon or small bucket.

Larvae

• 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 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.

Adult

q  Older juveniles and adults usually have a distinctive blue-green color, although sometimes they may take on a brownish hue.

q  Adult males are larger than the females, and the sexes are easily distinguishable.

q  The second walking legs or claws (chela) and the head region of males are larger than those of the females.

q   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.

q   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.

Larvae rearing tank preparation

The larvae rearing tank should be filled with treated 12 ppt water. The water is conditioned by circulating it through a biofilter for several days prior to the tank being stocked with newly-hatched larvae (Refer to page 19 for water treatment procedures). Add 10 ppm of EDTA before stocking with larvae. If the biofilter is new, a few handfuls of urea, or another ammonia salt, can be thrown into the tank to enhance the growth of denitrifying bacteria in the filter during the conditioning period.

Stocking larvae rearing tank

Stage I larvae are stocked at 100/1 in the larvae rearing tank. The remaining larvae should be transferred elsewhere. After ten days, the density of larvae can be adjusted to 60-801. By keeping all these parameters favourable and by controlling management accordingly, the end result can give up to 30-40 PL/l. In some cases, 60-100 PL/l have been produced. Fifteen to 20 PL/l seems to be about average.

Tank management

Maintaining stable water conditions in the tank is what ensures successful larvae rearing. This is much more critical in a recirculation system than in an open system hatchery. Salinity, temperature, ammonia and pH must all be kept within the limits necessary for the good health of the larvae.

Salinity control

The salinity of water in the larvae rearing tank should be maintained at 12 ppt up to the PL stage. However, ±2ppt does not affect growth of the larvae. Sudden fluctuations during water changes are to be avoided. Salinity can be checked and controlled by a hand refractometer. In a recirculation system, salinity may increase a little due to evaporation. Freshwater of the same temperature as the tank water should be added as needed to maintain the salinity at 12 ppt. Salinity should be checked every four or five days and freshwater added as required. If turbidity increases dramatically, an immediate water change is required. For this reason, treated 12 ppt water should always be available in the mixing tank.

Temperature regulation

The ideal temperature for rearing is 28-30°C.Temperature can be controlled by using immersion heaters. It has to be borne in mind that temperature below 24°Cand above 33°Care lethal to larvae. Fluctuations of temperature by more than 1°Care stressful and cause mortality. To heat the water in the tank, different types of heaters are available on the world market. About 400 watts per 1000 l of water is required. Water temperature should be checked five or six times daily if a thermostat is not being used. Covering the tank at night will help to reduce diurnal fluctuations. Larvae at all stages are attracted by light, but direct sunlight is harmful. It has been shown that growth and survival of larvae are improved in lighted tanks compared to dark tanks. Low, even illumination is preferable. Uneven illumination will cause ‘clumping’ of larvae in the brightest areas of the tank. Strong aeration counteracts the clumping tendency. Covering individual tanks is recommended to maintain temperature and to inhibit the spread of disease between tanks.

Fluorescent tubes can be used to evenly disperse light if tanks are inside a closed building.

Ammonia, Nitrite and pH control

Chemical changes take place in the water of the larvae rearing tank. Such changes take place because of the waste products of larvae and Artemia, dissolved fractions of feed supplied to the larvae, unused feed and spoilage of dead larvae. Some of these changes are very harmful. Un-ionized ammonia is the result of one such change. High pH increases the amount of un-ionized ammonia. Both nitrates and nitrites are harmful. Excessive nitrates increase mortality and retard growth, while 1.8 ppm of nitrite is lethal. The presence of un-ionized ammonia in very small quantities also induces mortality. The concentration of nitrites and nitrates in the water of the larvae tank should not exceed 1 ppm and 20 ppm respectively. The concentration of ammonia nitrite and nitrate is reduced and controlled by recirculating the water through the biofilter. Oyster or clamshells are good media for the biofilter, as the calcium carbonate content acts as a buffer against sudden changes in pH. The best pH range is 8.0 - 8.2.

Maintaining water quality

Cleanliness must be strictly maintained to ensure best results. Constant vigilance is required to prevent or control outbreaks of disease. Instruments and glassware should be kept separately for each tank, to prevent the transfer of disease between tanks. All tools and glassware should be disinfected by soaking them in a solution of potassium permanganate or formalin. After every larvae cycle, the tank should be washed and disinfected to prevent the growth of Zoo thumnium, Epistylis, hydroids and other disease organisms. The following precautions should be taken:

• Larvae should not be given feed in excess of their requirements.
• The walls of the tank should be cleaned with a soft brush every third day.
• After the first feed every day, aeration should be stopped and solid waste and dead larvae allowed settling at the bottom. These should then be removed by siphoning and the aeration turned on immediately thereafter.
• If the hatchery is located on a sea beach, only 50 per cent of the water needs to be exchanged daily. In a recirculation system, an even smaller proportion of tank volume needs be changed. A 20 per cent replacement on Day 10 and on Day 20 is beneficial.
• If for any reason the condition of the water deteriorates, or the movements of the larvae become weak, then 100 per cent of the rearing water should be changed.
• EDTA (usually 5-10 ppm) added to both hatching and larvae rearing tanks improves production.
• After completing a larvae rearing cycle, the side walls of the tank should be brushed well and kept moist for 24 hours with a strong solution of commercial bleaching powder. Formalin at 250 ppm may be used in place of bleaching powder. The disinfectant rinse should be followed by washing with clean water and drying for at least one day. Before starting work again, the tank should be rinsed with tap water.