Farming freshwater prawns

Figure 43Macrobrachiumrosenbergii farmscan be large (thisone was 70 ha)but need carefulproduction, marketingand businessmanagement forsustained success(Brazil)SOURCE: MICHAEL NEWDEFINING THE PONDChoosing its area and shapeIf you are going to use seining for harvesting, which is often practised in freshwater prawnfarming because of the necessity to cull out larger animals (and sometimes to separatefemales from males, when they have different values) before the final harvest, rectangularponds are the most suitable shape. The maximum width for this type of managementshould not be wider than the space through which a seine can be conveniently drawn fromone end of the pond to the other by manual labour. A convenient width is 30 m. In practice,of course, wider ponds can also be seined but not so efficiently as narrow ones. The lengthof the pond depends partly on the topography of the site and partly on the pond size andfarm layout chosen. It is best to standardize the width of ponds; otherwise a range of differentseine nets will be required for harvesting.The most easily managed pond sizes range between 0.2 ha and 1.6 ha, with mostfarms having ponds around 0.2-0.6 ha. If kept to a 30 m width, a 0.6 ha pond will be 200m long. Narrow ponds should be oriented so that the prevailing wind (which enhances thedissolved oxygen content of the water) blows down the long axis towards the drain end, tolessen the area of the pond bank subject to wave erosion.Large ponds are normally wider than 30 m and often drained for harvesting. If thetotal harvest is going to be taken at one time (batch management), the size of the pondshould be influenced by the maximum weight of prawns that the market will accept at onetime without price deflation. For example, if a quantity greater than 300 kg of freshwaterprawns would swamp your market and reduce prices it would be pointless to have a drainablepond greater than 0.15 ha in area (assuming a productivity of 2 mt/ha/crop).Information on pond construction is introduced in Muir and Lombardi (2000); detailsof construction techniques are provided in FAO (1995).Choosing its depthThe average depth of water in freshwater prawn ponds in tropical areas should be about0.9 m, with a minimum of 0.75 m and a maximum of 1.2 m. Deeper ponds (an average of1.2-1.4 m) are used in colder areas to maintain more stable water temperatures. However,deeper ponds are difficult to manage. Even if you have ponds of the recommended averagedepth you may have to drain or pump out part of the water to facilitate seining operationsat the deep end. In the cool season, the temperature of the water at the bottom of deepFRESHWATER PRAWNS86