Freshwater is normally used for rearing freshwater prawns from postlarvae to market size. Prawns will tolerate partially saline water (reports indicate that they have been experimentally cultured at up to 10 ppt; however, they do not grow so well at this salinity).
You could rear Macrobrachium rosenbergii in water which may be too saline to be drinkable or useful for irrigation. Water of 3-4 ppt salinity may be acceptable for the culture of M. rosenbergii, but do not expect to achieve results as good as those obtainable in freshwater.
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 ). Table 5 provides some criteria for water supplies for freshwater prawn nursery and grow-out facilities.
The water supply must be free from pollution, particularly agricultural chemicals. Prawn performance is likely to be adversely affected long before lethal levels are reached. However, the exact lethality of various chemicals is still being researched and it is not appropriate to list safe levels in this manual.
Those who wish to examine the status of this research may wish to consult Boyd and Zimmermann (2000), Correia, Suwannatous and New (2000) and Daniels, Cavalli and Smullen (2000).
As with hatcheries, the water must also be as predator-free as possible, though standards need not be quite so high. This may be achieved by screening (Figures 8a, 8b and 8c) or by the use of well water. Underground water, because of its chemical and microbiological quality and its lack of predators, is undoubtedly the preferred water source.
In practice, sites that only have access to surface water supplies (rivers, lakes, reservoirs, irrigation canals, etc.) are the most commonly used. However, you must be aware of the extra risk that their use brings. Screening the water supply helps to reduce the initial entry of predators but cannot clean up chemically polluted water or water containing disease organisms.
You should consider the location of other existing or planned freshwater prawn farms. You can then make an assessment of the risk that the water supplies of the new farm may be contaminated by the effluent from other farms.
If you are going to use surface water, constructing your farms close to a waterfall bringing water from a remote and unpolluted watershed or below the dam of a reservoir (though such water, if drawn from the epilimnion, may initially be high in hydrogen sulphide) would be ideal.
The minimum farm size for economic viability depends on several other factors but the quantity and continuity of the available water supply sets an absolute technical limit on the pond area of your farm, and on its potential productivity.
Water is required for four major purposes, namely filling ponds, compensating losses from seepage and evaporation, water exchange, and emergency flushing. When determining the amount of water available on a specific site for freshwater prawn farming you should take the rainfall pattern into account. This may be sufficient to replace or exceed evaporative and seepage losses, at least at some time during the year. An example of grow-out water requirements is provided in Box 3.
In addition to having enough water to fill the ponds it is, at the very minimum, necessary to have enough water available throughout the growing period to replace evaporative and seepage losses. Evaporative losses depend on solar radiation and wind and relative humidity and are therefore governed by the climatic features of the site.
Seepage losses depend on the soil characteristics of the farm area, mainly its permeability. Seepage losses may be small where the water table is high or where the water level of the pond is the same as in adjoining fields (e.g. in a paddy field area). However, in other cases, particularly where pond construction is poor, seepage losses can be very great. The quantity of water necessary for this purpose must be assessed locally and the cost of providing it is an important economic factor.
As ponds mature, ponds tend to ‘seal’ themselves, through the accumulation of detritus and algal growth, thus limiting seepage losses. Seepage losses can also be minimized by a number of techniques, including sealing the ponds with organic matter, puddling, compaction, laying out a ‘soil blanket’, applying bentonite, or lining them with polyethylene, PVC, or butyl rubber sheeting. Details of these procedures are provided in another FAO publication (FAO 1996).
There is no substitute for the site-specific determination of the water requirements for your farm but an example of water consumption needs for different sized farms, using a number of assumptions is given in Table 6. Techniques for measuring water resources are given in books on hydrology and agricultural water assessment such as ILACO (1981).
Methods for estimating seepage and evaporation losses and calculating water requirements are given in FAO (1981). Large-scale farms may wish to consult specialist contractors.
A supply of drinking water and waste disposal facilities are an added advantage to a freshwater prawn farm site but are not absolutely essential. Provision can be made onsite, for example by obtaining batch supplies of drinking water, sinking a borehole, or collecting and filtering rainwater.
However, if ice is going to be made, or prawns are to be processed and packed on site, a supply of high quality water, normally the equivalent of drinking (potable) water, is essential. Aqueous waste disposal from such activities can be routed to a septic tank, a waste lagoon, or a simple soak-away.
Author:
Michael B. New
