Topography And Access:
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. This will include transects, to evaluate slope and to determine the most economic ways of constructing ponds and moving earth.
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. In addition, ponds constructed on this type of site can be gravity filled (either naturally or by the creation of a dam) and gravity drained.
Where potential farm sites are steeper, or if gradients are irregular, 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.
The ideal site is rarely available, however. Many successful farms exist where the only feasible method to fill and drain the ponds is by pumping. Some sites, where ponds are excavated in flat, often seasonally flooded areas, may require higher pond banks for flood protection.
Prawn farming may be practised in rain-fed ponds but their productivity may be low. The level of productivity in grow-out ponds is governed by complex management factors, which are dealt with later in this manual. The cost of filling and draining ponds, which depend on the characteristics of the site, must be carefully assessed before the site is chosen.
Choosing Your Site: Climate
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 a peripheral 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 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 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.
Author:
Michael B. New
