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Fisheries > Culture Fisheries > Freshwater Prawns

Hatchery Management

Selection and Transporation of Broodstock

In the wild, berried females are most abundant around the beginning of the rainy season. When M. rosenbergii is reared in areas where the climate is sub-tropical or temperate, broodstock are typically obtained from ponds during the harvest at the end of the growing season and maintained indoors in environmentally controlled conditions during winter.

Broodstock constitutes an essential component in the hatchery phase. Freshwater ponds are required for the development of broodstock to have an uninterrupted supply of berried prawns to the hatchery. The advanced juveniles (>5 g) are stocked @ 1-2/m2 at a male female ratio of 1:4 in well-prepared earthen ponds. The prawns are fed with specially formulated broodstock diet (crude protein - 38%, lipid - 8%) twice daily @ 10% of the biomass for the first two months and subsequently at rates ranging from 5-2% of the biomass. The ponds are monitored and managed on a regular basis to maintain optimum water quality.

The prawns attain maturity in 2-3 months and females bearing advanced embryos are collected from these ponds regularly for the seed production purpose. This species breeds throughout the year under optimum climatic conditions (28-32ºC). However, in many parts of the country the peak breeding activity occurs during monsoon season, i.e. from July to November. For the operation of hatchery the berried prawns (bearing grey embryos) are collected at regular intervals from the broodstock ponds.

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Egg bearing (Berried) Females

The grey egg bearing females are collected using a cast net and are disinfected with 0.3 ppm of copper sulphate and released for hatching in brackishwater of 5‰. Normally average hatching fecundity is observed to be about 500 larvae per gram body weight of berried prawn. Hatching is completed within 2-3 days, if advanced berried prawns are collected with grey eggs on their pleopods. When hatchery do not have the provision of ponds, then grey berried females can be collected from either rivers or from grow-out ponds and can be kept for hatching in the larval rearing tanks itself. If it is not possible to get adequate numbers of berried females from nearby sources, then adult male and female prawns (>50 g) need to be stocked in broodstock tanks at 1:4 @ 1/m2 for breeding purpose.

The prawns should be fed with brood stock diet @ 5% of the biomass daily. Cleaning and water exchange @ 25-30% should be done daily. Berried females should be carefully selected. Choose animals that are obviously healthy and active, well pigmented, with no missing appendages or other damage, and carrying large egg masses. The ripeness of the eggs is also important. As the eggs ripen, their colour changes from bright orange to brown and finally to greyish-brown a few days before hatching. Those carrying brown to grey eggs are the best ones to bring into the hatchery, as their eggs will hatch within 2 or 3 days. It is best to ensure that you do this so that the whole larval batch is of a similar age. This will increase the efficiency of your feeding operations and reduce cannibalism. The number of females required depends on the volume of the hatchery tank to be stocked with larvae, and on the number of eggs carried by each female.

Transportation of the berried prawns to long distance is a hectic process. The prawns are held in tanks or double plastic bags, the rostrum of each animal is blunted with scissors or inserted into a plastic tube to prevent the bags being punctured. In addition, it is recommended that you shade the animals from light during transport; UV light may harm the eggs. Tying the chelipeds with rubber bands or covering them with plastic tubing also reduces the danger of the plastic bags being punctured.

A nutritionally complete diet is essential to promote superior egg production and quality. Commercially pelleted grow-out feeds can be used but need supplementation. Broodstock should be fed at a daily rate of 1-3% of total biomass, adjusted to match consumption. Half of the pelleted ration should be substituted with the equivalent amount of pieces of squid (or similar fresh feeds, such as clam meat), cut to the appropriate size, at least twice per week. 1 kg of a wet feed is roughly equivalent to 200 g of pelleted diet. The daily food ration should be given in two equal portions, normally in the early morning and late afternoon.

Hatchery phase

There are two basic types of nursery systems for freshwater prawns. The first type is known as the Flow-through system, which is effectively used in many parts of the world. The second type is the Recirculating system, which involves the use of biological filtration to conserve water and energy usage, reduce the demand for seawater or brine, and facilitate the establishment of inland hatcheries. They range from simple systems utilizable by small hatcheries to sophisticated systems used for research work and commercial hatcheries. The second type of hatchery described in this section uses a specific form of water recirculation and is known as the dynamic closed system. This is based on continuous circulation of the larval water through physical and biological filters to remove solid and nitrogenous wastes. This system can have individual biofilters for each larval tank or a common biofilter serving several larval tanks. The latter is naturally more risky because a failure in the system may affect a large number of larvae. However, the risk has to be balanced against the capital and maintenance costs of multiple filtration systems.

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Hatchery tanks

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Biofilter for hatchery tanks

Hatchery Management

Each cycle of operations in a freshwater prawn hatchery takes up to 40 days, including the time necessary to prepare for the next cycle. Careful attention to all aspects of hatchery management is essential to achieve success Water needs to be treated before it can be used in the hatchery. Usually, it is necessary to pass incoming water through some form of physical (gravel/sand bed) filter. B erried females are held in slightly brackishwater (~5 ppt) at 25-30°C and preferably at pH 7.0-7.2 until the eggs hatch. Temperatures below 25°C promote fungal growth on the eggs. Temperatures below the optimum also cause some eggs to drop and increase the time for egg development. Temperatures above 30°C encourage the development of protozoa and other undesirable microorganisms. Light does not seem to affect egg hatchability, although direct sunlight should be avoided.

About 1000 larvae are produced from each 1g of berried female weight. Berried females of 10-12 cm normally carry about 10000-30000 eggs. However, many eggs are lost through physical damage and cannibalism by the adult females during their transport from rearing ponds or capture fisheries, and some fail to hatch. A wide variety of feeds are employed by different hatcheries, including the nauplii of brine shrimp ( Artemia spp.), a freshwater cladoceran ( Moina spp.), fish eggs, squid flesh, frozen adult Artemia , flaked adult Artemia , fish flesh, egg custard, and commercial feeds.

Most freshwater prawn larvae do not feed on the first day (hatching day). However, it is recommended to provide some Artemia in the late afternoon of the first day because some larvae begin to eat early. From day 2 until day 4, feed Artemia five times per day, with the last and main feed in the evening. After that, you can gradually reduce the number of Artemia feeds per day until, by day 10, you are only giving Artemia at the evening feeding time. The evening meal should be given as late as possible (18.00-19.00). The amount of Artemia you give at each feeding time depends on your visual examination of the larval water. Freshwater prawn larvae do not actively search for food, which is why Artemia (which swim actively in the same part of the water column as the larvae) are such a valuable feed type.

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Ingredients for farm made larval diet

Ingredients Larval Feed
Fish Meal
100 g
Skimmed milk
250 g
Whole egg
10 eggs
Freshwater
250 ml
Wheat flour
250g
Vitamin C
5 tablets
Vitamin A & D
50 drops
Vitamin B Complex
5 tablets
Tetracycline
5 capsules
Calcicol
10 ml
(Source: Food and Agriculture Organisation, 2002)

Hatchery feeding schedule for freshwater prawn

Larval Stages Time
07.00 10.00 11.00 13.00 16.00
I to IV BSN - BSN - BSN
V BSN - ID/EC - BSN
VI to PL ID/EC ID/EC ID/EC BSN

BSN = Brine Shrimp Nauplii, ID = Inert Diet, EC = Egg Custard based diet

(Source: Food and Agriculture Organisation, 2002)

There should be about 3-6 Artemia/ml directly after feeding, depending on the age of the prawn larvae, and 1 Artemia/ml left in the water just before the next feeding time. A density of 3-6 Artemia/ml in a tank with 5 m 3 of water means that 15 to 30 million Artemia have to be added. As a rough guide, however, you can assume that 75 to 150 g of Artemia cysts will be required to produce the 15 to 30 million Artemia nauplii required for the daily feeding of a 5 m 3 larval tank initially stocked with 50 larvae/l and expected to provide about 25 PL/l. Normally one larval cycle in this size of tank will consume 1.25-2.5 kg of brine shrimp eggs.

Good hygiene is essential for hatchery success. Ideally, the same equipment should not be used for more than one tank. Thus each tank would have its own dedicated nets, siphon tubes, spare filters, etc. Water should never be transferred from one larval tank to another. Submersible pumps, which are often used for water transfer in hatcheries, should never be placed in the larval tanks because they are a potential source of disease transfer. Always drain your larval tanks by gravity or siphon and only use submersible pumps in water storage or mixing tanks. Disinfect all mobile equipment (buckets, siphoning tubes, nets, beakers, pipettes, etc.) on a daily basis. Dip them in a 500-ppm active chlorine solution, rinsing them very thoroughly with water and storing them dry. Between larval rearing cycles, routinely disinfect the larval tanks. Failure to do this usually results in massive blooms of organisms, such as Zoothamnium , Epistylis , hydroids, etc., which are harmful to the larvae.

Ozonisation or UV light can be employed to avoid the use of either chlorine or thiosulphate but, of course, these add to the capital costs and have not yet been noted in commercial freshwater prawn hatcheries. Several diseases affect freshwater prawn larvae.

An important disease in M. rosenbergii hatcheries, whose exact cause is unknown, is referred to as the ‘mid-cycle disease' (MCD). It is most noticeable in the middle of the larval rearing period (days 15-22 when the larvae are at stage VI-VII), when heavy daily mortalities may occur. Mortalities may even start to become obvious as early as day 10. The disease is recognisable by the larvae becoming bluish-grey and swimming slowly in a spiraling pattern, as well as by a reduced rate of consumption of Artemia and poor growth rate. Incidence of this disease can be reduced by cleaning, disinfecting and drying out hatchery equipment between cycles and taking special care in general hygiene throughout the larval cycle. If there is a severe outbreak of this disease, a full clean-up of the hatchery should be considered.

Another disease, which has mainly been noted to affect larvae, is known sometimes as the ‘exuvia entrapment disease' (EED) or as the ‘moult-death syndrome' (MDS) or as the ‘metamorphosis moult mortality syndrome'. The characteristic of this disease is that the larvae get trapped in the old exoskeleton (exuvia) during moulting. It is mostly noticed towards the end of the larval rearing cycle, especially at the moult, which occurs when, stage XI metamorphoses into the PL stage. The mortality rate at this point can be very high. The cause of EED is not known; it may have multiple causes. It may imply that the diet is nutritionally inadequate and requires enrichment. Difficulties in shedding the old exoskeleton during moulting have also been observed in juvenile and adult prawns.

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