Farming freshwater prawns

4.2 Hatchery managementEach cycle of operations in a freshwater prawn hatchery takes up to 40 days, including thetime necessary to prepare for the next cycle. Careful attention to all aspects of hatcherymanagement is essential to achieve success (the production of the maximum number ofhealthy postlarvae at the cheapest cost).WATER TREATMENTWater needs to be treated before it can be used in the hatchery. Usually, it is necessary topass incoming water through some form of physical (gravel/sand bed) filter (see Figure 30).If the incoming water is not filtered, or is still turbid, it may be necessary to allow the solidsto settle in one tank before transferring it to another tank for treatment. Mix the seawateror brine with freshwater to form 12 ppt brackishwater (see Table 4). Then treat thewater as in Box 6.Water drawn from underground sources may not need to be settled However, theremoval of protozoa and bacteria by chlorination, as indicated in Box 6, is still essential.Other forms of water treatment may be helpful. Some hatcheries filter the brackishwaterbefore use through 5 µm filters but most commercial hatcheries omit this step. Some add10 ppm of the chelating agent EDTA to larval rearing water to improve performance.Others use water which has a potentially unsuitable iron content (see Box 1).Water quality remains important, not only in the incoming supply but also withinthe hatchery itself. Monitor the water in your larval tanks frequently to see that its qualityis being maintained (Box 7). Simple field kits canbe purchased to check the parameters listed in Box7 but they are not specified in this manual becausethey are standard water quality items. For smallflow-through hatcheries it is not practical to installfacilities for the other types of analytical work,Treatment ofespecially where they involve the analysis of seawateror brackishwater. Samples of water for thebrackishwateranalysis of other parameters, such as hardness,metals, pesticide residues, etc., should be sent toALLOW IT TO STAND so that any sediments governments, universities or private laboratorieswill settle. It is essential to remove as much who have the facilities and staff to deal with them.of the suspended solids as possible, otherwisethe chlorination that follows may be partially or available in Boyd (1979).Further reading on water quality and analysis istotally ineffectiveIf you are using a recirculation system, adjustChlorinate the brackishwater with 5 ppm of active the salinity and temperature to 5-7 ppt and 28 tochlorine 7 .31°C, allowing the system to stabilize before stocking.This salinity allows larvae to be stocked direct-Allow the water to stand for one day.Remove the residual chlorine by vigorous aeration ly from the hatching tank without acclimation. Thefor 6 hours before use (note: sodium thiosulphate can salinity in the culture tank should then be increasedbe used to remove the residual chlorine more quickly to 12 ppt. Neither artificial brackishwater nor freshwatershould be added through the biofilter tankbut its use is not recommended because it also mayprove toxic to larvae. Vigorous aeration for 6 hours is once the biofilter substrate has been added. Its bacteriaare sensitive to sudden changes in adequate).tempera-BOX 67 Both dry and liquid sources of chlorine vary considerably in their chlorine content from product to product and from batch to batch. It is best to determinethe level of chlorine in each batch before use, to ensure that you are actually using the quantity of chlorine that you think you are adding. Themethods for determining the level of chlorine in commercial bleaching powder or liquid bleach are contained in Annex 4, Table 6.FRESHWATER PRAWNS54