Water exchange is a common practice in shrimp farming. Average daily water exchange rates of 5% to 30% are common. Despite wide use, the benefits of water exchange are poorly understood, and it is often used ineffectively (Boyd & Fast, 1992).

Pond water salinity often increases in dry seasons. The influence of evaporation on salinity may be quite drastic. The water level in ponds is commonly maintained with just enough water to replace evaporation and seepage. Seepage does not affect salinity. Evaporation causes salinity to increase because salts are not lost when water evaporates (Boyd & Fast, 1992).

Water exchange is also a means of improving other water quality conditions. If dissolved oxygen concentration in a pond is low and the water in the supply canal has a high dissolved oxygen concentration, high water exchange rates can improve dissolved oxygen concentrations in the pond. Likewise, excessive nutrients, plankton and toxic metabolites can be flushed from ponds through water exchange. Influent water should enter ponds at the surface and effluent pond water should be discharged from a drain located near the pond bottom and on the opposite side of the pond. The most beneficial means of exchanging water in a pond is to first drain out the volume to be exchanged and then pump or let in an equal volume of high quality water (Boyd & Fast, 1992).

Water should not be exchanged if water quality in the pond is good because nutrients and plankton needed for shrimp production will be flushed out. It is wasteful to exchange water in ponds where fertilizers have just been applied. Also, pond water sometimes may be of better quality than water in the water supply canal. In this event, water should not be exchanged (Boyd & Fast, 1992).