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Prawn Forming

Prawn Forming

Prawn Cultivation

Indian aquaculture has been evolving from the level of susbsistence activity to that of an industry. This transformation has been made possible with the development and standardization of many new production and associated techniques of input and output subsystems. In recent years aquaculture has created great enthusiasm and interest among entrepreneurs especially for shrimp farming in coastal areas. Shrimp farming is capital intensive activity and uncontrolled mushrooming growth of it has led to outbreak of diseases and attributed environmental issues calling for closure of shrimp farms.

Although India has vast freshwater resources they are not fully exploited except for carp culture in limited scale. Fresh water fish culture employing composite fish culture technology has become popular for use in large number of tanks and ponds in the country. To meet the raw material required by the processing units for export demand there is urgent need to expand our production base. In addition it is always stressed that there is a need to utilise our natural resources productively to ensure the much needed food security.

Species

Biology

Macrobrachium rosenbergii is found in inland freshwater areas including lakes, rivers, swamps, irrigation ditches, canals and ponds, as well as in estuarine areas. This species requires brackishwater in the initial stages of their life cycle (and therefore they are found in water that is directly or indirectly connected with the sea). Mature male prawns are considerably larger than the females and the second chelipeds are much larger and thicker. The head of the male is also proportionately larger, and the abdomen is narrower. The head of the mature female and its second walking legs are much smaller than the adult male. A ripe or ‘ovigerous' female can easily be detected because the ovaries can be seen as large orange-coloured masses occupying a large portion of the dorsal and lateral parts of the cephalothorax.

The life cycle of M. rosenbergii can be summarized as follows. The mating (copulation) of adults results in the deposition of a gelatinous mass of semen on the underside of the thoracic region of the female's body (between the walking legs). Successful mating can only take place between ripe females, which have just completed their pre-mating moult (usually at night) and are therefore soft-shelled, and hard-shelled males. In tropical areas these coincide with the onset of the rainy season. Within a few hours of copulation, eggs are extruded through the gonopores and guided by the ovipositing setae (stiff hairs), which are at the base of the walking legs, into the brood chamber. During this process the semen attached to the exterior of the female's body fertilizes the eggs. The eggs are held in the brood chamber and kept aerated by vigorous movements of the swimmerets.

The length of time that the eggs are carried by female freshwater prawns varies but is not normally longer than three weeks. The number of eggs that are laid also depends on the size of the female. Female prawns of M. rosenbergii are reported to lay from 80000 to 100000 eggs during one spawning when fully mature. Egg incubation time averaged 20 days at 28°C (range 18-23 days).

Freshwater prawn eggs of this species are slightly elliptical, with a long axis of 0.6-0.7 mm, and are bright orange in colour until 2-3 days before hatching when they become grey-black. This colour change occurs as the embryos utilize their food reserves. As the eggs hatch, rapid movements of the abdominal appendages of the parent disperse the larvae. Freshwater prawn larvae are planktonic and swim actively tail first, ventral side uppermost (i.e. upside down). M. rosenbergii larvae require brackishwater for survival. The larvae go through 11 distinct stages before metamorphosing into post larvae. Stage I larvae (zoeae) are just under 2 mm long (from the tip of the rostrum to the tip of the telson). Larvae swim upside down by using their thoracic appendages and are positively attracted to light. By stage XI they are about 7.7 mm long. Newly metamorphosed post larvae (PL) are also about 7.7 mm long and are characterized by the fact that they move and swim in the same way as adult prawns. They are generally translucent and have a light orange pink head area.

Site Selection for Hatcheries and Nurseries

The site requirements for hatcheries and nurseries, which are normally associated with each other, are similar.

Availability of Quality Water

The hatchery and nursery should be located inland where there is ample supply of good freshwater. Saline water required for larval development can be transported and mixed with freshwater to attain the desired salinity. The quality of intake water, whether it is saline or fresh, is of paramount importance for efficient hatchery operation. Water quality is thus a critical factor in site selection. Hatchery sites should preferably be far from cities, harbours and industrial centres, or other activities, which may pollute the water supply. In all cases, water supplies need careful analysis during site selection, to determine their physical, chemical, and biological characteristics, and the extent to which these may vary daily, seasonally, or through other cycles.

Special care is needed in hatcheries that are situated in or near areas where the use of pesticides, herbicides, and fertilizers is intensive. Ideally, freshwater should be obtained from underground sources. The brackishwater for use in M. rosenbergii hatcheries should be 12-16 ppt, should have a pH of 7.0 to 8.5, and contain a minimum dissolved oxygen level of 5 ppm. High levels of heavy metals, such as mercury (Hg), lead (Pb) and zinc (Zn), should also be avoided, since these are most likely to be caused by industrial pollution.

Soil Characteristics

The ideal soil for freshwater prawn culture should be clay-silt mixture or sandy loam comprising of 60% sand and 40% silt with good water retention capacity. There must be enough soil available for pond construction, whether the ponds are to be excavated or pond banks are to be erected above ground. Although supplemental food is given to freshwater prawns reared in earthen ponds, a considerable amount of their food intake is from natural sources. It is therefore preferable to site the farm where the soil is fertile, as this will reduce the need and costs of fertilisation. Freshwater prawn ponds should be constructed on soil, which has good water retention characteristics or where suitable materials can be economically brought onto the site to improve water retention.

Pervious soils, which are very sandy or consist of a mixture of gravel and sand, are unsuitable unless the water table is high and surrounding areas are always waterlogged. Soils, which consist of silt or clay, or a mixture of these with a small proportion of sand, normally have good water retention characteristics. Peaty soils are not suitable. The clay content should not exceed 60%; higher clay content soils swell when moist and crack during the dry season, thus making repairs necessary.

 

On completion of their larval life, freshwater prawns metamorphose into post larvae (PL). From this point onwards they resemble miniature adult prawns and become mainly crawling rather than free-swimming animals. When they do swim it is usually in a normal (dorsal side uppermost) way and in a forward direction. Rapid evasive movement is also achieved by contracting the abdominal muscles and rapid movement of the tail. Post larvae exhibit good tolerance to a wide range of salinities, which is a characteristic of freshwater prawns.

Post larvae begin to migrate upstream into freshwater conditions within one or two weeks after metamorphosis and are soon able to swim against rapidly flowing currents and to crawl over the stones at the shallow edges of rivers and in rapids. In addition to using the foods available to them as larvae, they now utilize larger pieces of organic material, both of animal and vegetable origin. Post larval freshwater prawns are omnivorous can also be cannibalistic.

Stocking

The type of pond preparation to be adopted before stocking is based on the type of culture and its intensity and nature of the culture pond. Liming of the pond assumes great importance than in the case of freshwater fish culture.

The application of fertilisers is restricted in case pelletised feed is used.

However, occasionally cow dung, single super phosphate, urea etc. can be applied on assessing the productivity.

The stocking density normally varies from 4000 to 50000 PL/ha depending on the type and intensity of the management practices.

The culture system may be monoculture or polyculture with carps. In case of polyculture with carps the more pond depth is preferred at 4-5 feet.

In case of polyculture the stocking density of prawn may vary from 2500-20000 post larvae. The carp fingerlings may be of the order of 500-2500 nos.

Nursery may be incorporated where the post larvae obtained from hatcheries could be reared for a period of 4-5 weeks till they attain 40-50 mm or 1-3 gm. 

In order to get desired production, feeding, aeration, water exchange, periodic monitoring should be continued. The quality and type of feed is based on culture system.

Macrobrachium with its omnivorous feeding habits can make use of a variety of feeds from common wet feed made from rice bran and oil cake to scientifically formulated pelleted feed.

The rate of feeding is determined by the stage of growth of prawn, water quality, density of stock and other manuring practices. Generally the feeding rate my be 5% of the body weight.

The duration of culture varies from 6 to 12 months depending on the type of culture practice. Generally in monoculture the culture period may be 6-8 months under monoculture and 8-12 months under polyculture.

The average growth of prawn may range from 50 gms to 200 gms depending on the duration, density, water quality, feeding etc. The survival rate may range 50% to 70% depending on the type of management practices.

Feeding

It is necessary to maintain an adequate phytoplankton density, to provide cover and control the growth of weeds in freshwater prawn ponds. This is done by encouraging the growth of phytoplankton. However, it is often unnecessary to fertilize, because this is rapidly achieved by the feeding regime. However, ponds built in a sandy-clay soil may require fertilization for this purpose. Where necessary, 25 kg/ha/month of triple superphosphate will keep the water green. Benthic fauna are very important features in the ecosystem of freshwater prawn ponds, forming part of the food chain for prawns. Fertilisation to encourage the development of benthic fauna is therefore recommended. Animal manures have been used for this purpose (e.g. 1000-3000 kg/ha of cattle manure).

The types of feed used in freshwater prawn farming vary widely and include individual animal or vegetable raw materials and feed mixtures prepared at the pond bank; both of these are generally referred to as ‘farm-made feeds'. In addition, commercial feeds designed for freshwater prawns are available. Freshwater prawns are omnivores and, so far as is known at present, their nutritional requirements are not very demanding. Some farmers utilize commercial feeds designed for marine shrimp in freshwater prawn nurseries or during the first few weeks of the grow-out phase when prawns are stocked as PL. Marine shrimp feeds have a much higher protein content than is needed for freshwater prawns, so cheaper commercial feeds that have either been specifically designed for freshwater prawns or for a species of fish (e.g. catfish) must be used in grow-out ponds stocked with nursery-reared juveniles, or substituted as soon as possible in those stocked with PL. The prawns are fed daily with formulated pellet diet (2-3 mm size) @ 10% of the biomass initially and then reduced to 3% of the biomass towards the end of the culture period.

Feed formula for Freshwater prawn

Ingredients

Feed 1 (%)

Feed 2 (%)

Fishmeal 20 -
Shrimp Head meal - 30
Soya meal 9 5
Rice bran 45 35
Coconut oil cake 20 20
Tapioca starch 5 9
Mineral premix 1 1
Total 100 100

There can be no exact general recommendation for daily feeding rates, because these depend on the size and number of prawns (and, in a polyculture system, fish) in the pond, the water quality, and the nature of the feed. The feed should be broadcasted in the pond as mentioned above. Spread the feed around the periphery of the pond in the shallows, which are good feeding zones. Check trays 3-4 nos may be kept in different corners of the pond to check the consumption of food.

Pond Management

Nursery pond management

The preferred stocking density in the nursery pond is 20/m2. Post-larvae (8-10 mg) may be fed with pellet diet (crude protein 35%; lipid 8%) in crumble form @ 100% of the biomass during the first fortnight and further reduced to 50% in subsequent period. In the absence of pellet diet a mixture of groundnut oil cake (powdered) and rice bran may be given as feed. The feed should be broadcasted in the pond twice daily preferably in the morning and in the late evenings. In nursery ponds approximately 10% of the pond surface may be covered with floating weeds with dense root system such as Eichhornia sp. to improve the survival rate of post-larvae. The weeds should be kept inside a PVC or bamboo frame to avoid their spreading in the pond. Aeration is provided for ~8 h/day.

A fortnight after stocking sampling of post-larvae may be done to observe the growth using cast net or fry net. During nursery rearing water temperature may be checked twice daily. pH, dissolved oxygen, transparency and depth may be checked once every week and to be maintained in optimum ranges. Loss of water due to seepage and evaporation should be compensated by water addition at least once every fortnight. Nursery rearing may be done for 45-60 days. At the end of rearing period the juveniles (>1.0 g) are collected by dewatering the pond and transferred to grow-out ponds

Juvenile prawns can be harvested by seining your ponds two or three times with a 5 to 6 mm mesh seine, or by emptying them completely. Polypropylene boxes or tanks filled with water from the nursery pond and kept aerated, can be used to transport the juveniles to the grow-out ponds if they are close by. There are some advantages in grading the juveniles into two or three groups, depending on their average weight, before stocking them into separate grow-out facilities. This decreases competition in grow-out ponds by reducing Heterogeneous Individual Growth (HIG) and increases productivity.

Some mortality (10-20%) will occur soon after PL are stocked, even when the conditions are ideal. Total survival from stocking until removal from the nursery ponds should be at least 75%. The weight of the prawns at the end of the outdoor nursery period should be about 0.8-2.0 g, but the time taken to reach those sizes will depend on local conditions.

Post Harvest Handling

In general, the value of harvested product depends on its quality. Speed during and after harvesting, getting the prawns on ice and out of the sun, and care in handling to prevent physical damage, will all reap valuable dividends.

Handling prawns to be sold fresh

If you intend to sell your prawns fresh (instead of selling them alive or frozen) you will need to keep them very cool. You should not place live prawns straight onto ice; this results in a slow decline in body temperature, causes stress, and accelerates the deterioration process, which occurs after death. To kill a batch of 50 kg of prawns, for example, immerse them in 50 l of water and 80 kg of ice for 30 minutes.

Finally, you should wash them in chlorinated water (5 ppm active chlorine). After killing, remove prawns from the cold water and immediately place them in isothermal boxes, with alternate layers of ice and prawns, placing ice in the first and last layers.

Handling prawns to be sold frozen

If prawns are to be sold within 5 days of harvest, which is considered to be their maximum practical refrigerated shelf life, freeze them immediately. Freezing at temperatures below -10°C is essential; storage at -20°C or below is recommended; storage at -30°C is ideal.

To avoid physical damage to the muscle structure of the prawns, it is recommended that the freezing temperature passes from -1°C to -5°C as rapidly as possible (not more than 2 hours). This decreases the production of ‘drip' (leak) at the moment of thawing, and keeps the prawns looking and tasting the same as before freezing.

If you freeze them more slowly, it will cause large ice crystals to form between the cells of the animals and increase ‘drip'. Keeping prawns frozen on-farm is generally not good practice, except on very large farms where specialist equipment has been installed.

Harvesting

Basically there are two methods of harvesting: cull-harvesting drain harvesting. Harvesting time depends partly on growth rate and the size of animals you want to sell. This, in turn, depends on your market requirements. It also depends on the pond management technique chosen. Cull-harvesting is used to harvest market-sized animals from the pond at intervals and removes the faster growing prawns. In tropical ponds cull-harvesting usually starts 5-7 months after PL have been stocked, or sooner if juveniles have been stocked. After cull-harvesting commences, you should totally seine each pond once per month or partially seine it twice per month (i.e. seine half the pond twice per month or all of it once per month). Take out the market-sized animals and sell them. Keep the smaller ones and soft-shelled animals in the pond for further growth. After about 8-11 months, drain the pond and sell the whole harvest.

Cull Harvesting

In this technique a seine net is pulled through the pond to remove market-sized animals. The net may be a simple seine or one constructed especially for the purpose, usually made of monofilament nylon, and provided with floaters, sinkers and sometimes a bag. The size of the seine you use depends on the size of pond you are using it for.

Nets which are 2.5 m high with a length equivalent to 1.6 times the pond width are suggested. The mesh you choose depends on the size of animal to be marketed. Although stretched knot mesh sizes of as low as 0.7 inches (1.8 cm) are sometimes used where there is a market for small prawns, the usual recommended mesh size is 1.5-2 inches (3.8-5 cm).

Care must be taken to ensure that the bottom of your seine is kept on the bottom of your pond. If you do not, many prawns will escape beneath it. Preferably you should pull the seine down the long axis of the pond (this is why rectangular ponds of 30 m maximum width are preferred) so that the ends of the net are pulled along the banks of the pond. Seining different halves of the pond once every two weeks avoids disturbing the whole of the pond at once.

Drain Harvesting

The method and efficiency of drain harvesting depends on the design of the pond. As with any other method of harvesting, speed is important and harvesting should start very early in the morning while the temperature is cool. You can partially draw down the pond water level during the night before harvesting commences.

Site Selection for Hatcheries and Nurseries

The site requirements for hatcheries and nurseries, which are normally associated with each other, are similar.

Availability of Quality Water

The hatchery and nursery should be located inland where there is ample supply of good freshwater. Saline water required for larval development can be transported and mixed with freshwater to attain the desired salinity. The quality of intake water, whether it is saline or fresh, is of paramount importance for efficient hatchery operation. Water quality is thus a critical factor in site selection. Hatchery sites should preferably be far from cities, harbours and industrial centres, or other activities, which may pollute the water supply. In all cases, water supplies need careful analysis during site selection, to determine their physical, chemical, and biological characteristics, and the extent to which these may vary daily, seasonally, or through other cycles.

Special care is needed in hatcheries that are situated in or near areas where the use of pesticides, herbicides, and fertilizers is intensive. Ideally, freshwater should be obtained from underground sources. The brackishwater for use in M. rosenbergii hatcheries should be 12-16 ppt, should have a pH of 7.0 to 8.5, and contain a minimum dissolved oxygen level of 5 ppm. High levels of heavy metals, such as mercury (Hg), lead (Pb) and zinc (Zn), should also be avoided, since these are most likely to be caused by industrial pollution.

Soil Characteristics

The ideal soil for freshwater prawn culture should be clay-silt mixture or sandy loam comprising of 60% sand and 40% silt with good water retention capacity. There must be enough soil available for pond construction, whether the ponds are to be excavated or pond banks are to be erected above ground. Although supplemental food is given to freshwater prawns reared in earthen ponds, a considerable amount of their food intake is from natural sources. It is therefore preferable to site the farm where the soil is fertile, as this will reduce the need and costs of fertilisation. Freshwater prawn ponds should be constructed on soil, which has good water retention characteristics or where suitable materials can be economically brought onto the site to improve water retention.

Pervious soils, which are very sandy or consist of a mixture of gravel and sand, are unsuitable unless the water table is high and surrounding areas are always waterlogged. Soils, which consist of silt or clay, or a mixture of these with a small proportion of sand, normally have good water retention characteristics. Peaty soils are not suitable. The clay content should not exceed 60%; higher clay content soils swell when moist and crack during the dry season, thus making repairs necessary.

Other requirements for Hatchery sites

In addition to having sufficient supplies of good quality water, a good hatchery site should also have:

•  A secure power supply, which is not subject to lengthy power failures. An onsite emergency generator is essential. 
•  Have good all-weather road access for incoming materials and outgoing PL; 
•  Have access to food supplies for larvae; 
•  Employ a high level of technical and managerial skills; 
•  Have access to professional biological assistance from government or other sources; 
•  Have its own indoor/outdoor nursery facilities or be close to other nurseries and 
•  Be as close as possible to the market for its PL. In the extreme case, it should not take more than 16 hours of total transport time from the furthest farm to the market.

Site Selection for Outdoor Nurseries and Grow-Out Facilities

The success of any nursery facility or grow-out farm depends on its access to good markets for its output. Its products may be sold to other farms (in the case of nurseries), directly to the public, to local markets and catering facilities, or to processors or exporters. The needs and potential of each type of market need to be considered.

It also important to consider other factors to ensure success, including the:

•  Suitability of the climatic conditions; 
•  Suitability of the topography; 
•  Availability of adequate supplies of good quality water; 
•  Availability of suitable soil for pond construction; 
•  Maximum protection from agricultural and industrial pollution; 
•  Availability of adequate physical access to the site for the provision of supplies and the movement of harvested animals; 
•  Availability of supplies of other necessary inputs, including postlarval and/or juvenile prawns, equipment, aquafeeds or feed ingredients, and power supplies; 
•  Availability of good skilled (managerial) and unskilled labour;

Topography

Farms must be close to their market so the road access must be good. Large farms will need to have local access for heavy trucks be able to reach the farm easily, for the delivery of supplies and the efficient collection of harvested prawns.

A survey is necessary, to assess the suitability of a site from a topographical point of view. It is important to minimize the quantities of earth to be shifted during pond construction. Flat or slightly sloping lands are the most satisfactory. The ideal site, which slopes close to 2% (2 m in 100 m), allows good savings on earth movement. Care should be taken to ensure that pond sizes and alignments allow efficient construction, and at the same time permit good access and effective water supply and drainage.

Climate

The meteorological records such as temperature, the amount and seasonality of rainfall, evaporation, sunlight, wind speed and direction, and relative humidity should be studied for site selection. Avoid highly unstable meteorological regions. Strong storms and winds increase the risks of flood and erosion damage, and may lead to problems with transport access and power supply.

Temperature is a key factor. Seasonal production is possible in semi-tropical zones where the monthly average air temperature remains above 20°C for at least seven months of the year. The optimum temperature range for year-round production is between 25 and 31°C, with the best results achievable if the water temperature is between 28 and 31°C. The temperature of the rearing water is governed not only by the air and ground temperature but also by solar warming and the cooling effects of wind and evaporation. The rate by which pond water is exchanged and the temperature of the incoming water are also important considerations.

Rainfall, evaporation rates, relative air humidity and wind speed and direction also need to be investigated. Ideally, evaporation losses should be equal to or slightly lower than rainfall input, to maintain an approximate water balance. Mild winds are useful to promote gas exchange (oxygenation) between water and the atmosphere. However, strong winds can increase water losses by evaporation and may also generate wave action, causing erosion of the pond banks. Avoid areas where it is constantly cloudy because this makes it hard to maintain a steady water temperature, as it interferes with solar penetration. Periods of cloud cover of several days' duration may also cause algal blooms to crash, which in turn lead to oxygen depletion.

Nursery Phase

The nursery can be either indoor or outdoor. The selection of sites for indoor nurseries should follow the same pattern as for hatcheries. Site selection for outdoor nursery facilities should be similar to that for grow-out ponds.

Holding Tanks

After rearing freshwater prawns in hatchery, hold them until ready for stocking in ponds. Concrete tanks of 50 m3 are convenient for holding postlarvae (PL) prior to transport for stocking in ponds. Use nets suspended from floats in the tanks to increase the surface area available to the PL but this may make the normal operations of feeding, cleaning etc. more difficult.

Water Management

Water quality and supply

Freshwater is normally used for rearing freshwater prawns from postlarvae to market size. Water of 3-4 ppt salinity may be acceptable for the culture of M. rosenbergii. The reliability of the quality and quantity of the water available at the site is a critical factor in site choice. However, as in the case of hatchery water supplies, the absolute ‘ideal' for rearing sites may be difficult to define; a range of water qualities may be generally suitable. As for hatchery water, the level of calcium in the freshwater seems to be important. Growth rate has been reported to be lower in hard than in soft water. It is recommended that freshwater prawn farming should not be attempted where the water supply has a total hardness of more than 150 mg/l (CaCO3).

Water quality requirements for prawn nursery and grow out

Variables Recommended range
Temperature (oC) 28-31
pH 7.0-8.5
Dissolved oxygen (ppm) 3-7
Salinity (ppt) <10
Transparency (cm) 25-40
Alkalinity (ppm) 25-60
Total hardness (ppm) 30-150
Ammonia (ppm) <0.3
Nitrite (ppm) <2.0
Nitrate (ppm) <10
Boron (ppm) <0.75
Iron (ppm) <1.0
Copper (ppm) <0.02
Manganese (ppm) <0.10
Zinc (ppm) <0.20
Hydrogen sulfide (ppm) Nil