Coastal Shrimp Aquaculture in Thailand: Key Issues for Research

!use water which is almost fresh (Funge-Smith 1997). This is achieved by gradualacclimatisation of larvae to progressively less saline waters (F. Dieberg, pers. comm.;Funge-Smith 1997).Before the occurrence of white spot virus, postlarvae were usually released intoponds that had not been treated by chemicals prior to the release of the larvae. Nylonnets were commonly used to prevent the entry of fish that could prey on them. Wherewater was treated, lime, zeolite, dolomite, benzalkonium chloride, or chlorine wereused. Manure may also have been added to the pond to encourage the growth ofplankton and other micro-organisms to provide food for the postlarvae (NACA1994a).However, since the white spot virus, management of ponds and intake water hasbecome more critical. Large intake reservoirs, occupying one-third to one-half of thefarm area, have been constructed on many farms to store and treat water before it isused. Treatment commonly involves the use of chlorine and other agents to reducevectors of the disease (Flegel 1996; Funge-Smith 1997). The use of intake treatmentponds may have economic advantages in areas with water quality problems. NACA(1994a) found that farms in southern Thailand using settling ponds had an averageincome of 66,110 baht/rai (pond area)/crop compared with 36,169 baht/rai/crop onfarms without them.The amount of water used by each farm is dependent on the quality and quantityof the water available and on the individual management strategies of the farms. It isaffected by stocking density, stage of shrimp growth, feeding regime, and food typeand quality. The greater the stocking density, the greater the amount of waterrequired to remove the proportionally larger quantity of wastes from the system(Phillips et al. 1993). The percentage of the pond volume exchanged each day wasaveraged for a sample of intensive farms in the districts of Ranot and Hua Sai accordingto the stage of growth of the crop (Table 4). As would be expected, the volumeand frequency of exchange increased over the rearing period. The study also calculatedthe volume of water required for ponds of particular sizes with different stockingdensities (Table 5).It should also be noted that the quantity of water used by grow-out farmers hasbeen substantially reduced in recent years following the occurrence of white spot.Farmers consider introduction of new water into the farm to be a potential risk andhave adapted management practices to minimise it.Table 4.Rate and volume of water exchange for intensive shrimp ponds in the districts ofRanot and Hua Sai (NACA 1994a).Stage of crop Frequency of water exchange % of pond volume exchanged(days)Month 1 12 4Month 2 6 15Month 3 4 17Month 4 3 20In contrast, shrimp hatcheries use relatively little water. According to Kungvankijet al. (1986), backyard hatcheries can produce 1 million postlarvae per year in 20 m 3+'