Shrimp Farming and the Environment - Library

In more immediately practical terms, the safe and effective use of chemicals in aquaculture has recentlybeen reviewed by GESAMP (1997) and is also presented in relation to specific diseases by AAHRI(Chanratchakool, Turnbull, et al. 1996).DiseaseSome of the diseases that trouble the shrimp farming industry are directly caused by environmentalproblems, while a number of other diseases are triggered or spread more effectively by the stressinduced by environmental problems. None of the shrimp diseases are known to be pathogenic tohumans. In recent years, shrimp farming has been afflicted with outbreaks of viral diseases that havegreatly undermined profitability and sustainability of operations. Based on a detailed survey conductedin 1993–94, the Network of Aquaculture Centres in Asia estimated the total losses in 12 countries inSouth and Southeast Asia to amount to U.S.$143 billion dollars (ADB/NACA 1995).Disease outbreaks have led to the collapse of shrimp farming in some places, including Taiwan,Republic of China; parts of Thailand; the east coast of India; and China. In China, the collapse has ledto a shift away from shrimp to other species such as finfish, mollusks, and crab, some of which aregrown in polyculture.Viral diseasesMore than 15 different viruses have been identified for Penaeid shrimp over the past 20 years. Many ofthe known viruses infect larvae and juveniles, and they can be species-specific. Shrimp may becomeless resistant under conditions of stress, such as overcrowding, water temperature fluctuation, lowoxygen levels, or high levels of pollutants (Lundin 1996).Taura SyndromeIn the Western Hemisphere, the most damaging disease at present is Taura Syndrome Virus, referred toas TSV or just TS (Rosenberry 1996). Attacking shortly after shrimp are stocked, it kills from 40 to90% of the juveniles in a pond. It is called “Taura Syndrome” because it first appeared on the TauraRiver, about 25 km southeast of Guayaquil, Ecuador. It has also been called “Little Red Tail” (LaColita Roja) because the fan tail and body of affected shrimp turn pale pink.Although TSV may have been active for a number of years earlier, it was first noticed in shrimp farmsin Ecuador in 1992. Several farms were affected by it, then it disappeared quickly but reappeared inMarch 1993. At that time, it became a major epidemic, killing farm-raised shrimp throughout the Gulfof Guayaquil.Taura spread from Ecuador to Colombia as early as 1993. In 1994, most farms in Honduras andGuatemala were affected as well, and by 1995 there were reports of the disease in Mexico. In May1995, TSV hit Texas shrimp farms and killed 90% of the crop.In 1993, 1994, and 1995, Ecuadorian shrimp farmers continued to produce large amounts of shrimp,although many areas were infected with TSV. Profits were lower, because the farmers stocked theponds with twice the ordinary number of juveniles.There is now evidence that the white-leg shrimp (Penaeus vannamei) has become resistant to TSV. Inmid-June 1995, survival of wild-caught seed ranged from 50 to 60%, while survival of hatcheryproducedseed was 20 to 30%. This was still double the survival rates experienced in 1993–94.There have been no reports of Taura in Costa Rica, Panama, Venezuela, or Nicaragua.White Spot Virus DiseaseIn the Eastern Hemisphere, White Spot Virus Disease is the most common and serious shrimp diseaseaffecting shrimp farms (Rosenberry 1996). In Thailand, attempts to eradicate the disease have so farfailed. White spot was also probably responsible for the major shrimp farming disasters in Taiwan,Republic of China, in 1987–88 and in China in 1993. It has caused problems in Bangladesh, India, andVietnam, and is probably established everywhere in Southeast Asia.29