Prawn Farming At Home

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Where Do Prawns Come From?

All freshwater prawns (like other crustaceans) have to regularly cast their ‘exoskeleton’ or shell in order to grow. This process is referred to as moulting and is accompanied by a sudden increase in size and weight. There are four distinct phases in the life cycle of the freshwater prawn, namely eggs, larvae, post larvae (PL) and adults. The time spent by each species of Macrobrachium in the different phases of its life cycle (and its growth rate and maximum size) varies, not only specifically but according to environmental conditions, mainly temperature.

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 (referred to as a spermatophore) 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 hardshelled males. All of the various types of males are capable of fertilising females but their behaviour is different. Detailed descriptions of the mating process are given in Ismael and New (2000) and Karplus, Malecha and Sagi (2000). Under natural conditions, mating occurs throughout the year, although there are sometimes peaks of activity related to environmental conditions. In tropical areas these coincide with the onset of the rainy season, whereas in temperate areas they occur in the summer.

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Within a few hours of copulation, eggs are extruded through the gono pores 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 eggs are fertilized by the semen attached to the exterior of the female’s body. The eggs are held in the brood chamber (stuck to the ovigeroussetae) and kept aerated by vigorous movements of the swimmerets. This is in contrast to penaeid shrimp, whose fertilized eggs are released into the sea, where they hatch. 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 which are laid depends also on the size of the female. Female prawns of M. rosenbergii are reported to lay from 80 000 to 100 000eggs during one spawning when fully mature. However, their first broods, (i.e. those which are produced within their first year of life), are often not more than 5 000 to 20 000.

Females normally become mature when they reach 15-20 g but berried females have been observed as small as 6.5 g (Daniels, Cavalli and Smullen 2000). Under laboratory conditions, where a breeding stock of both males and females was kept, it has been noted that egg incubation time averaged 20 days at 28°C (range 18-23 days). Ovaries frequently ripened again while females were carrying eggs. Pre-mate intermoults were separated by as little as 23 days (i.e. females on some occasions hatched two batches of eggs within one-month period). It is unlikely that this would happen under natural conditions but it does show the potential fecundity of the animal.

As the eggs hatch, a process which is normally completed for the whole brood within one or two nights, the larvae (free-swimming zoeae) are dispersed by rapid movements of the abdominal appendages of the parent. Freshwater prawn larvae are planktonic and swim actively tail first, ventral side uppermost (i.e. upside down). M. rosenbergii larvae require brackish water for survival. Those which hatch in freshwater will die unless they reach brackish water within a few days. There are a number of microscopically distinct stages during the larval life of freshwater prawns, which lasts several weeks.Individual larvae of M. rosenbergii have been observed, in hatchery conditions, to complete their larval life in as little as 16 days but reaching this stage may take much longer, depending on water temperature and other factors. The importance of this observation is fully discussed later in the manual. Larvae eat continuously and, in nature, their diet is principally zooplankton (mainly minute crustaceans), very small worms, and the larval stages of other aquatic invertebrates.

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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 fan. Postlarvae exhibit good tolerance to a wide range of salinities, which is a characteristic of freshwater prawns.

Postlarvae 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. They can climb vertical surfaces and cross land, provided there is abundant moisture available. 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 and, as they grow, their natural diet eventually includes aquatic insects and their larvae, algae, nuts, grain, seeds, fruits, small molluscs and crustaceans, fish flesh and the offal of fish and other animals. They can also be cannibalistic. Further reading on this topic maybe found in Ling.

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How To Start A Prawn Hatchery

The site requirements for hatcheries and indoor nurseries, which are normally associated with each other, are similar. In this section of the manual, reference to hatcheries therefore includes indoor nurseries.

NEEDS FOR GOOD QUALITY WATER

Although the larval stages of freshwater prawns require brackish water for growth and survival, hatcheries do not have to be located on coastal sites. Prawn hatcheries can be sited on inland sites. There, the necessary brackish water can be obtained by mixing locally available freshwater with seawater or brine (and sometimes artificial seawater) which has-been transported to the site. Two decades ago, when the original FAO manual was written, most hatcheries operated on flow-through systems.

Many still do so but the establishment of inland hatcheries, the costs of obtaining and transporting seawater or brine, and increasing concerns about the discharge of saline water in inland areas have encouraged some operators to minimize water consumption through partial or full recirculation systems. Inland hatcheries have the advantage that they can be sited wherever suitable freshwater is available and their market (namely outdoor nurseries and grow-out facilities) is close by. Where to site a hatchery is therefore not only a technical but also an economic consideration.

Also Read: How do you raise freshwater prawns at home?

This involves balancing the costs of transporting seawater and brine, or using recirculation, against the advantages of an inland site. Prawn hatcheries, regardless of type, require an abundant source of freshwater as well as seawater or brine. The quality of intake water, whether it be 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.

Due to the extra problems and dangers involved, it is generally recommended that freshwater prawn hatcheries should not be sited where the only source of water is surface water. However, this guidance has not always been observed. The minimum requirement during site evaluation should be to carry out watershed surveys and water analyses, especially for pesticides and oil spill residues. In coastal areas, it may be possible to draw good quality water from sub-surface layers, usually with freshwater overlying more saline water. The ideal site, where wells sunk to different depths provide both freshwater and seawater, is rare, although it is sometimes possible to make good use of groundwater sources, which are usually cleaner and less liable to become contaminated. The quality of water depends on the soil materials. In coastal areas with underlying coral rock, hatcheries can often get good quality seawater, free of pollution or harmful protozoa and bacteria. If sites with borehole seawater are not available, direct access to a sandy beach with mixed sand particle size can be selected. On this type of site a shallow beach filter of the type described in Annex 2 can be utilized. Muddy areas are not so suitable, but a larger filter may be used, provided it can be cleaned out periodically.

Many freshwater prawn hatcheries utilize surface supplies for both freshwater and seawater. Often, seawater can be drawn from areas where the salinity is 30 to 35 ppt, usually through a rigid pier off-take in the sea or a flexible buoyed system. Crude screening can be used to prevent the entry of the larger flora and fauna but this alone is not sufficient to protect the larvae from disease and parasitical problems. The use of unfiltered water will almost certainly result in disaster, so additional filtration is essential. Brine, sometimes used instead of seawater for inland hatcheries to minimize transport costs, can be obtained from salt evaporation pans. The brine, which is often between 80-100 ppt salinity but can be as high as 180 ppt, can be diluted with freshwater to form brackish water (in theory, the higher the salinity of the brine used, the better; this is because the sudden osmotic shock which occurs when brine and freshwater are mixed together may reduce the numbers of bacteria and parasites present in the original supplies). Some hatcheries obtain freshwater pumped or fed by gravity from surface supplies such as rivers or irrigation canals. This practice exposes the hatchery to severe variations in water quality and particularly to water contamination from agricultural chemicals.

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 where hatcheries are situated in or near areas where the use of pesticides, herbicides, and fertilizers is intensive. Ideally, freshwater should be obtained from underground sources, though some of these may be unsuitable because of high levels of iron and manganese, which are lethal to prawn larvae. Methods of reducing the levels of these ions are provided later in this section of the manual. City tap water is also normally suitable, provided it is vigorously aerated for 24-48 hours before use to remove residual chlorine, but may be too expensive to use. Well water should also be aerated, by cascading for example, to bring its dissolved oxygen level up to, or near to saturation point.

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