Direct seawater pumping is the simplest means of obtaining saltwater. In coastal freshwater prawn hatcheries, topography and tidal movement need to be considered. If the coastal characteristics indicate the existence of sufficient sub-surface water of suitable salinity, wells should be used. Another alternative is to draw water by means of perforated pipes protected by a 150 ^m nylon screen buried approximately 1 m deep and connected to a suction system, or to use the simple shallow beach wells, as described by New (2002). Water sources are more thoroughly discussed in section 5.3.
Pipework is required to distribute seawater and freshwater from storage tanks to mixing tanks and from there to supply brackishwater to each larval rearing and Artemia nauplii hatching tank. The optimum salinity for hatching Artemia can vary from 5 p.p.t. to full-strength seawater, depending on the source of cysts, thus full-strength seawater may also be piped to Artemia nauplii hatching tanks. A supply of freshwater, as well as brackishwater, needs to be piped to each larval tank in flow-through systems in order to allow for adjustments in salinity following siphoning, to replace evaporated water and to clean the tanks. All distribution systems need to be carefully designed to avoid 'dead' areas, where stale water can accumulate; 'ring-main' systems (continuous closed circuits) are one way of avoiding this.
Drainage of larval tanks is usually by gravity and the drainage canals should carry the drained water away without any contamination of the water sources. It is convenient to treat this water with sodium hypochlorite before discharge, in order to minimise environmental damage.
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