In his May presentation to members of the National Fisheries Institute Shrimp Council, Mr Chamberlain updated the EMS status of primary production areas and passed on recommendations for management methods to reduce the impacts of EMS. The update comes about two months after GAA launched a case study to identify the shrimp-farming practices that prevent the manifestation of EMS. The case study will act as a foundation for recommendations to the aquaculture community for better shrimp-farming practices.
EMS Status
Mr Chamberlain said the incidence of EMS in China varies among regions. In early 2014, EMS appeared at a low rate in eastern and southwestern Guangdong, and Guangxi provinces, but manifested at moderate to high rates in the Pearl River and Zhangjiang regions. Farms in northern China had not been stocked.
Some operations have engineered deep ponds with self-cleaning bottoms that also use heavy water exchange to eliminate sludge. The presence of covert mortality nodavirus and microsporidean parasites, other pathogens that affect the hepatopancreas organs of shrimp and cause slow growth and size variation, have made clear identification of EMS more challenging in the country.
Despite the presence of EMS, recent high shrimp prices continued to fuel rapid farm expansion in Viet Nam, Mr Chamberlain said. Now that prices have dropped to US $5.00/kg, fewer farms are stocking, and postlarvae sales are dropping.
Thailand was again hit hard by EMS. First quarter estimates showed only 30,000 mt of production versus 100,000 mt last year. Coming off three months of low temperatures and facing continued failure rates of over 30 per cent in the first 40 days, many farms are not expected to restock for a time.
In Malaysia, production remained down overall, although concerted efforts at the large-scale farm run by Agrobest are yielding rising results.
Mr Chamberlain said Mexico’s first outbreaks of EMS came at Nayarit in the center of Sinaloa and a couple of farms in Sonora. New farms starting in the south and on the Gulf of Mexico seem to have escaped EMS so far. Head-on shrimp production of 55,000 to 60,000 mt is projected for the country.
India faces an unsure prognosis regarding EMS. Test results have been inconsistent and inconclusive, so the country’s producers should be considered free of EMS at the present time.
Expert Observations
Mr Chamberlain shared a number of observations and recommendations made by members of an expert committee on EMS formed by GAA. For example, those working with EMS have identified varying toxicity in the multiple strains of the Vibrio bacteria that cause the disease. Although test methods are improving, EMS losses continue to be confused with mortalities caused by viral diseases such as white spot syndrome and Taura syndrome.
EMS is thought to be transmitted vertically (on the outside of eggs) from broodstock to postlarvae, and can also transfer to shrimp via water, cannibalism, feces, plankton, macro-organisms, birds and biofilms. Once established in the environment, EMS is difficult to control, Mr Chamberlain said.
Importantly, it has been found in China that antibiotics are not effective against EMS. Sensitivity tests have shown the bacteria responsible for EMS outbreaks have already developed resistance to the full range of antibiotics. Research in Mexico shows that pathogenic Vibrio strains have nearly the same resistance profiles as non pathogenic strains. However, Mexican researchers agree that antibiotics are not effective, because they do not effectively reach the pathogen which colonizes chitinous surfaces.
Recommendations
• EMS-free broodstock are needed. Selective breeding for resistance to EMS would involve challenging families and selecting those with best survival as parents for the next generation.
• Improve farm practices. To eliminate EMS and other pathogens from water, establish a balanced microbial population, stock with healthy postlarvae and closely manage water and bottom quality. Disinfection with chlorine or ozone eliminates multiple pathogens. To create a mature microbial community, probiotics and polyculture can help condition water. Other suggestions were to maintain light to moderate bioflocs, avoid overfeeding and remove sludge regularly.
• Employ a nursery phase. Nurseries hold young postlarvae until they are larger and more robust, while also confirming they are free of EMS. Ten to 20 days in a nursery raceway, tank or net pen allows important physical and medical evaluations.
• Improve farm infrastructure with biosecure intensive ponds. Small, deep ponds covered with plastic or bird nets allow more manageable disinfection and feeding, as well as better control of water quality, bioflocs and sludge. Higher yields (30-50 mt/ha) can cover the extra investment.
• Identify feed additives that reduce the incidence of EMS. These could include quorum-sensing inhibitors, essential oils or immunostimulants.
• Integrated farm management is needed. Fragmented systems do not provide needed controls at each step in the production process. Zone management would avoid farm sitings with shared inlet and discharge canals, and consider the carrying capacity of ecosystems. Access to well-equipped local labs would provide better detection of EMS.
Global EMS Survey
Building on earlier studies coordinated by the Global Aquaculture Alliance that helped identify the cause of EMS and other elements of the disease, GAA is launching a new online survey to collect additional information on the EMS status of farms in affected areas, as well as the practices they apply to combat the disease.
The comprehensive survey -- funded by the World Bank Allfish project, the Seafood Industry Research Fund of the National Fisheries Institute and C.P. Prima of Indonesia -- will initially be available in English at www.gaalliance.org. Versions in other languages will be added to better allow those in Asian and Latin American countries to participate.
Based on responses to the survey, a select number of farms will be chosen to receive in-depth site audits and diagnostic testing to clarify what practices are most effective in managing EMS. In combination with the survey results, these findings will be distributed by GAA to help identify the common denominators of proper management and promote the adoption of better shrimp-farming practices industry wide.
