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December 13, 2013
Practical Advise on Dealing with EMS
This article on combatting EMS by Karunanithi Muthusamy (below) appeared in the November/December 2013 issue of AQUA Culture AsiaPacific.
Breeding: Over the past five years there has been a push to produce postlarvae with superior growth rates. Usually this is done by selecting and breeding strains or families that exhibit these qualities. When we focus on fast growth, there may be a negative effect on other desirous traits such as disease resistance. Dr. Chalor Limsuwan (email firstname.lastname@example.org), a professor in the Department of Fishery Biology at Kasetsart University in Thailand, and his team think that inbreeding of fast-growing shrimp could be contributing to the EMS outbreaks because fast-growers are weak and susceptible to disease.
Broodstock: Reports from hatcheries indicate that broodstock that have been ablated for spawning are now dying prematurely, sometimes even during the spawning process. Previously, ablated broodstock would spawn several times before they were retired. Limsuwan recommends natural spawning without eyestalk ablation to produce shrimp that may be slower growing, but healthier and stronger.
Hatchery: During the heydays of giant tiger shrimp (Penaeus monodon) farming, hatcheries provided quality control documents which contained detailed and prompt reports on the quality of the postlarvae they were supplying. In subsequent years, however, when whitespot syndrome virus (WSSV) began destroying crops of tiger shrimp, farmers shifted to farming specific pathogen free (SPF) white shrimp (P. vannamei). With the shift to white shrimp, the use of quality control documents seems to have disappeared. I recommend that hatcheries resume the practice of providing quality control documents to farmers.
For quality control, postlarvae should be checked against the 13 criteria in the table. I recommend that farmers stock postlarvae older than 12 days because at this stage the hepatopancreas and gills are well developed.
Inferior postlarvae have played a part in almost 80% of EMS occurrences in Malaysia. Stocking densities of eggs, zoea, mysis and postlarvae at hatcheries are higher than optimum levels. I believe that this is stressful for the postlarvae, which may be able to survive in the hatchery because of the controlled conditions, but quickly succumb to disease when they leave the hatchery.
Commercial hatcheries should invite farmers to visit their facilities and check postlarvae quality before making purchases. Farmers should perform freshwater immersion tests and formalin stress tests at the hatchery and later at the farm to compare the differences and gain a better understanding of the postlarvae selection criteria. Before stocking ponds, hatcheries and farms should communicate and exchange technical information. Hatcheries and farms need to work closer together to prevent the outbreaks of disease. Once the postlarvae are transferred to growout ponds, various changes in pond parameters such as low minerals, unstable plankton transparency, low dissolved oxygen, variations in temperature, pH, salinity and unpredictable weather could result in the already stressed postlarvae quickly succumbing to infections. Some growout practices recommend that the pH of the water should be maintained at lower than 7.6. This is not ideal for shrimp culture and may promote the growth of pathogens. At the same time, hydrogen sulfide (H2S) becomes more toxic at low pH. Newly stocked shrimp molt very frequently and grow quickly. This is a very vulnerable time for them, and maintaining the correct minerals in the water is extremely important. Minerals should be maintained at optimum levels or slightly higher. When the minerals fall below 50% of the recommended amount and the shrimp have soft shells, the likelihood of EMS occurring is higher. Soft shells are a symptom of EMS. It is important that pH is maintained between 7.8 and 8.1. Another essential practice to improve the immunity of the postlarvae is to supplement their feeds with immunostimulants, multivitamins and minerals.
Pond Preparation: During pond preparation, plowing pond bottoms is encouraged to increase the soil pH and soil temperature and to eliminate H2S. Focus on maintaining good water quality throughout culture; manage feed to prevent overfeeding; and closely monitor the mineral content of the water. Many farmers who have instituted these practices have been able to continue farming without EMS through the first 70 days of growout.
Growout: Postlarvae in the pond are under constant attack by bacteria (Vibrio, Pseudomonas, Ralstonia and Rhodococcus). Postlarvae succumb easily to diseases when they have poor immunity. EMS is probably caused by multiple links among pathogens (for example, Vibrio parahaemolyticus and V. harveyi) that release toxins and unidentified toxins from blue green algae. Hydrogen sulfide plays a role here as well. In combination with vibriosis, the conducive factors for EMS are low water pH (< 7.6), low mineral content after stocking, over feeding during the first 30 days of culture and the presence of H2S. This combination of factors could directly contribute to EMS attacks, especially among weak, stressed and molting shrimp.
In earthen ponds, pathogens usually thrive in the organic material or black soil a few inches below the bottom of the pond, which has a low pH (<5.5) and low temperature (< 20°C). In fully lined HDPE ponds, many farmers are not aware of anaerobic conditions underneath the liner where there is accumulation of toxic H2S gas, also known as the “silent killer”. In older HDPE ponds, this toxic gas slowly leaches through the fine holes in the HDPE and causes stress in the shrimp.
Gut pH: Fluctuations in gut pH affect the optimum function of the hepatopancreas. Normally the shrimp gut has a neutral pH of around 7.0. P. vannamei feeding behavior is more aggressive than P. monodon and many toxic substances including Vibrios and toxic blue green algae can enter the gut system and may cause an increase in pH. This may lead to improper digestion, inefficient nutrition absorption and storage of essential nutrition in the hepatopancreas. To eliminate such Vibrios, add probiotic additives, such as “Pond Plus” (Novozymes) or yeast based additives to lower gut pH.
Keeping EMS Under Control: Farms on Penang Island, Malaysia, that following the above recommendations and protocols, have not encountered EMS recently, and on the mainland, a few farms in Penang are reporting successful harvests. In southern Malaysia, there are farms in Pekan, Johor Bahru, Pontian and Kota Tinggi that are reporting good harvests. In Sarawak, East Malaysia (the north coast of the island of Borneo) only one farm has been able to overcome EMS. It recently achieved an average production of about 10 metric tons/0.6 hectare. In Sabah, East Malaysia (the northeast tip of the island of Borneo), two farmers in Kudat have not experienced any EMS. One has been able to achieve production of 7 tons/0.5 hectare and the other has achieved 20 tons/0.6 hectare. There are also two farms in Kunak, Sabah, with no EMS and average production of 6 tons/0.5 hectare.
What To Do When EMS Strikes: When shrimp show signs of dying, the farmer should take the following steps immediately. Switch on aerators for 5-8 days. Stop feeding for 5 to 8 days, depending on the mortality numbers. Apply water disinfectants (such as Remedor Aquatic, Bayer) to reduce Vibrio, viral and fungal organisms. Apply lime (CaCO3) to maintain pH above 7.8 in the morning and less than 8.1 in the evening. Daily pH difference should be less than 0.3 and total and bicarbonate alkalinity should be increased to more than 130/130ppm. Finally, apply mineral products such as Exel Aqua Basic (EAB, Bayer) to maintain optimum mineral levels at night.
On the second day of an EMS outbreak, apply beneficial bacteria at double the normal dosage to improve water quality. Monitor all water quality readings and take measures to correct those that are out of their normal range. Do the same thing on the third day. On the fourth day, apply a soil based probiotic (Pond Dtox, Novozymes) at double the normal dosage, while continuing to monitor all water quality readings and making appropriate adjustments. Resume feeding when there are no dead shrimp in the feeding trays. Start at 30% of the usual feeding rate and and increasing it until you reach 500 grams of feed per 100,000 postlarvae. Feeds should be mixed with feed additives such as gut probiotics, minerals, immune boosters and lipid enhancers.
Information: Karunanithi Muthusamy, the author of this article, is technical support manager at Syndel Asia Sdn.Bhd, Malaysia. He has over 25 years of experience in farming and technical support for farmers in Malaysia, Indonesia and Papua New Guinea. He is passionate about helping the shrimp industry.
Information: Zuridah Merican, Editor/Publisher, AQUA Culture AsiaPacific, 3 Pickering Street, #02-36 Nankin Row, China Square Central, Singapore 048660 (phone +603-2096-2275, fax: +603-2096-2276, mobile +6012-205-3130, email email@example.com, webpage http://www.aquaasiapac.com).
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