Farming freshwater prawns

cal 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, irrigationcanals, etc.) are the most commonly used. However, you must be aware of the extra riskthat their use brings. Screening the water supply helps to reduce the initial entry of predatorsbut cannot clean up chemically polluted water or water containing disease organisms.You should consider the location of other existing or planned freshwater prawnfarms. You can then make an assessment of the risk that the water supplies of the newfarm may be contaminated by the effluent from other farms. If you are going to use surfacewater, constructing your farms close to a waterfall bringing water from a remote andunpolluted watershed or below the dam of a reservoir (though such water, if drawn fromthe epilimnion, may initially be high in hydrogen sulphide) would be ideal.The minimum farm size for economic viability depends on several other factors butthe quantity and continuity of the available water supply sets an absolute technical limiton the pond area of your farm, and on its potential productivity. Water is required for fourmajor purposes, namely filling ponds, compensating losses from seepage and evaporation,water exchange, and emergency flushing. When determining the amount of water availableon a specific site for freshwater prawn farming you should take the rainfall patterninto account. This may be sufficient to replace or exceed evaporative and seepage losses, atleast at some time during the year. An example of grow-out water requirements is providedin Box 3.In addition to having enough water to fill the ponds it is, at the very minimum, necessaryto have enough water available throughout the growing period to replace evaporativeand seepage losses. Evaporative losses depend on solar radiation and wind and relativehumidity and are therefore governed by the climatic features of the site. Seepage lossesdepend on the soil characteristics of the farm area, mainly its permeability. Seepagelosses may be small where the water table is high or where the water level of the pond isthe same as in adjoining fields (e.g. in a paddy field area). However, in other cases, particularlywhere pond construction is poor, seepage losses can be very great. The quantity ofwater necessary for this purpose must be assessed locally and the cost of providing it is anBOX 3Grow-out water requirementsTO FILL A 0.2 ha pond withan average water depth of0.9 m requires 10 000 x 0.2x 0.9 = 1 800 m 3 of water. Since it isusually desirable to be able to fill thepond within 12 hours, it follows that itmust be possible to extract up to1 800 ÷ 12 ÷ 60 = 2.5 m 3 (2 500 L) perminute from the water source for thispond. Normally it is only necessary tocompletely fill a drained pond after arearing cycle is completed and thepond has been drained and treated,that is, once every 6-11 months.There will also be times when,because of poor pond water quality,you may find it necessary to flush thepond and replace a substantial proportionof the water while prawnsare growing in it. However, it is veryunlikely that it will be necessary foryou to fill more than one pond at thesame time, if you have a small farm.Thus, for example, five 0.2 ha pondswould therefore not require a maximumwater supply five times largerthan one 0.2 ha pond.FRESHWATER PRAWNS22