This is another fundamentally important issue. You should study the meteorological records to determine temperature, the amount and seasonality of rainfall, evaporation, sunlight, wind speed and direction, and relative humidity. 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. As far as possible, do not site the farm in an area which is subjected to severe periodic natural catastrophes, such as floods, typhoons, landslips, etc. If you decide to site your farm in an area subject to floods, you will need to make sure that the banks of individual ponds are higher than the highest known water level at that site, or you will need to protect the whole farm with aperipheral bank.
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. This occurs, for example, in China and some southern States of continental USA. For successful year-round farming, sites with large diurnal and seasonal fluctuations should be avoided. The optimum temperature range for year-round production is between25 and 31°C, with the best results achievable if the water temperature is between 28 and31°C. The temperature of the rearing water is governed not only by the air and ground temperature but 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. However, in some locations this balance changes seasonally.
There may be cooler high-rainfall periods during which water can be stored in deeper ponds, and hotter high-evaporation periods in which water supplies decrease. In these areas, it is still possible for you to produce one or more crops by adjusting production plans. 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.
Apart from the dangers of water-supply contamination, you should not site your farm in an area where the ponds themselves are likely to be affected by aerial drift of agricultural sprays; prevailing wind direction should therefore be taken into account.
Constructing ponds adjacent to areas where aerial application of herbicides or pesticides is practised is also undesirable. Freshwater prawns, like other crustaceans, are especially susceptible to insecticides.
CHOOSING YOUR SITE: WATER QUALITY AND SUPPLY
Freshwater is normally used for rearing freshwater prawns from post larvae 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).
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).