Farming freshwater prawns

Figure 28Power supplies arenot always reliable.Loss of aerationcan quickly causedevastation in ahatchery. Thishatchery has notonly installed twoelectrically-drivenblowers (one as aback-up) but hasalso provided apetrol engine sothat the drive beltscan be rapidlychanged if thepower fails(Thailand)The individual tank water inlets can be arranged so that they can be turned awayfrom the tank and water can be flushed to waste before it is directed into the tank. Thisprevents water from entering the larval rearing tanks that may be stagnant or very warmbecause it may have come from pipes exposed to sunlight. Another way of ensuring thatthis cannot happen is to fix a short length of flexible hose to each water inlet and alwaysallow the water to flush to waste for a minute or two before letting it flow into the tank.The choice of pump size depends on the scale and design of each specific hatchery. Asnoted earlier, specific hatchery design is not a part of this manual. In sizing, pumps shouldbe chosen which will fill the appropriate tank at the maximum rate required, not the averagerate. There is nothing more annoying than a slow-filling tank due to pump under-sizing.Copper and zinc are toxic to freshwater prawns but there should be no problem in theuse of pumps containing alloys of these two metals (which are often chosen, particularly forseawater pumping, because of their corrosion resistance) where the water passes throughthe pump only once. Pumps which are submerged in water (submersible pumps), or whichform part of recirculation systems, must have those parts in contact with water made of aninert material, such as plastic. Air lift pumps (see Figures 13a and 13b) are also extremelyuseful for recirculating water or for transfer of water from one tank to another. In allcases, pump sizes should be standardized as far as possible to minimize the number ofstandby pumps necessary. Makesure that you can replace an outof-orderpump simply and quicklyand keep adequate spares inworking order on site at all times.Maintaining equipment such aspumps, blowers and generatorsin good general working conditionis critically important. Theseitems should have a weekly functionalcheck.In recirculation systems,proper water flow rates and goodcirculation is essential for adequatewaste removal. All equipmentmust be capable of supportingthe maximum water flow rateneeded during the larval cycle.Ideally, the total water volume inthe larval rearing tanks shouldcirculate through the filter anaverage of at least 10 times perday (1 000%) but pump sizingSOURCE: HASSANAI KONGKEOshould be based on the maximumflow demand. When the larvalstocking rate is high, the water may need to be passed through the filter at a turnover rateof 70 to 100% per hour. Thus a 5 m 3 larval culture system would require a system capableof providing a water flow of 5 m 3 /hour. This can best be done through the use of airliftpumps (see Figures 13a and 13b). All pumps, filters and disinfection systems must be sizedto provide this maximum flow rate. Useful information on pumps and pumping for aquacultureis given in Wheaton (1977).CHAPTER 447