Farming freshwater prawns

Figure 53Where thetopography of thesite makes itfeasible, supplyingwater by gravitykeeps the dissolvedoxygen level high(Brazil)Figure 54Supplying waterabove the pondwater level providessome oxygenation,while grassminimizes erosionof the bank (Brazil)SOURCE: JULIO VICENTE LOMBARDI, REPRODUCED FROM NEW AND VALENTI(2000) WITH PERMISSION FROM BLACKWELL SCIENCESOURCE: WAGNER VALENTIfer of water from one pond to another is not recommended, because it means poorer waterquality conditions in the second (and subsequent) ponds and brings the risk of disease transfer.Ideally, water should be distributed in pipes or open channels by gravity if the topographyof the site allows it (Figure 53). Similarly, inlet pipes or channels should be constructedabove the water level in the ponds so that the incoming water falls onto the surface of thewater (Figure 54). This may be achieved by pumping the water supply to an elevated channel,if this is economically feasible. The water inlet is normally placed at the shallow end ofthe pond, opposite the discharge point. The inlet channels (or pipes) and the outlet pipesmust be correctly sized according to the water demand and draining needs of each pond.Table 12 gives the water discharge capacity of concrete pipes under various pressure heads.Detailed information on these topics is provided in other FAO manuals (FAO 1992b, 1995).The flow of water into each pond must be controlled by valves, weirs, stop-logs orplugs (Figure 55). Detailed instructions on building these structures are given in FAO(1992b). While gravity supply, elevated water inlets and lack of cross-contamination ofwater between adjacent ponds represent the ideal, many freshwater prawn farms existwhich do not comply with these recommendations. Many farms (due to site, technical, orWater discharge capacity (in m 3 /hr) of concrete pipesTABLE12 under various pressure headsINSIDEDIAMETEROF PIPE (CM)PRESSURE HEAD (CM)5 10 15 20 25 100 20020 67 95 116 134 15025 105 149 182 210 235 432 57630 151 214 262 302 33835 206 291 357 412 460 684 1 152NOTE: THE WATER DISCHARGE CAPACITY OF PIPES INCREASES WITH THE PRESSURE HEAD (VERTICAL DISTANCE BETWEEN WATER SURFACE ABOVE ANDCENTRE OF LINE OF PIPE BELOW).SOURCE: DERIVED FROM FAO (1995)CHAPTER 693