Coastal Shrimp Aquaculture in Thailand: Key Issues for Research

!no significant difference between the bacterial population in the canals supplyingboth farms which had failed and those which had successfully operated extensive andsemi-intensive systems (Kakorkin and Sunaryanto 1994). Research by the AquastarCompany showed that postlarvae supplied to their farms by hatcheries commonlyhave pathogenic organisms associated with them (Fegan 1994).Pathogens cause disease if shrimp are stressed. Stress alters the internal biochemicalbalance of the animals, leaving them susceptible to infection (Shariff and Subasinghe1992). In shrimp farms, factors which cause stress to shrimp are overcrowdingand poor water and sediment quality. In recognition of the role of poor water qualityin the development of disease in shrimp, farmers often change their water as soon asthey see evidence of poor health in their animals. This triggers moulting, a phaseduring which the shrimp are particularly sensitive to the state of the surroundingenvironment. Problems with the sediment quality can then induce worsening of theexisting disease syndrome, and may lead to death. This sequence of events may beresponsible for a number of the crop failures that farmers attribute to intake of pollutedwater. Rather than an acute problem triggered by properties of the exchangedwater, the problem was a chronic one, the result of poor pond condition (J. Turnbull,pers. comm.).Disease diagnosis is difficult since ill health in shrimp cannot be diagnosed easilyby observation of external signs or behaviour. Laboratory techniques are most usefulbut even these are not always successful unless the disease condition is severe (Turnbull1993). Laboratory diagnosis techniques often take 3–4 days, by which time thedisease may be well advanced or have resulted in mortalities. New faster techniques,such as polymerase chain reaction (PCR) techniques, are being developed.Research efforts are improving the knowledge and management of particular diseases.Yellow head disease caused a loss of an estimated 20–25% of production inThailand in 1992 and early 1993. A coordinated response by industry, academic andgovernment sectors helped to limit the spread of the disease. The causal organismwas identified by research at the National Institute for Coastal Aquaculture as a newvirus, subsequently named yellow head baculovirus (Fegan 1994).All research dealing with disease in shrimp recognises the need for a wellmanaged pond system to reduce the incidence of disease and to allow treatments andcontrol measures to be effective. The greatest progress in disease management can bemade by the use of good management and husbandry to improve the culture environmentas emphasised by Chanratchakool et al. (1994).Local Socioeconomic Impacts of Shrimp FarmingPositive economic and social impactsShrimp farming has generated employment both on farms and indirectly throughservicing industries. These industries include hatcheries, feed plants, transportationoperations, and freezing and cold storage plants, as well as manufacturers of productssuch as paddle wheels for pond aeration, containers for shrimp storage and transport,hatchery tanks, water pumps, chemicals for water treatment, and chemotherapeutics+#