How to Prevent Early Mortality Syndrome (EMS) in Shrimp Farms

A new/emerging shrimp disease known as early mortality syndrome (EMS) or acute hepatopancreas necrosis syndrome (AHPNS) has been reported to cause significant losses among shrimp farms in China (2009), Vietnam (2010) and Malaysia (2011). Recently in Thailand (2012), it has been reported to affect shrimp in the eastern and some provinces in the southern Gulf of Thailand. The disease affects both Black tiger shrimp (Penaeus monodon) and Pacific white shrimp (Litopenaeus vannamei) and is characterized by mass mortalities (reaching up to 100 % in some cases) during the first 20-30 days of culture (post-stocking in grow-out ponds).  Affected shrimp consistently show an abnormal hepatopancreas, which may be shrunken, pale color, soft shells and whitist muscle. Moribund shrimp sink to bottom.

Different species of bacteria, were isolated from diseased shrimp, bacteriophages and gregarine parasites are also found as well.  However, up to dates scientists have not been able to pinpoint the exact cause of EMS because Koch’s postulate cannot be performed. When the micro-organisms found in the diseased shrimps’ bodies are isolated, cultured and used to inoculate healthy shrimp, the healthy shrimp do not get EMS. In addition, when diseased shrimp from a pond with EMS are placed in a tank with healthy shrimp in the laboratory, the healthy shrimp do not get EMS, so it is not a serious infectious disease like white spot syndrome virus or yellow head virus.

Because most of the shrimp die within 30 days post stocking, it is likely that a major cause of EMS is unhealthy postlarvae. Large amounts of vibrio bacteria are found in the hepatopancreas of moribund shrimp, and often EMS occurs in ponds that were not well prepared at times when there is heavy and continuous rain. An extensive study of ponds that experienced EMS and ponds in nearby areas with no EMS, undertaken from late 2011 to the end of 2012, resulted in some information that can help shrimp farmers prevent EMS.

1/ Select healthy postlarvae

Shrimp farmers should stock at least post larvae stage 10 (PL 10) if you are using normal saltwater in the ponds, but if the water has salinity of less than  15 ppt, you should start with postlarvae that are larger than PL 10. The postlarvae should have completely formed hepatopancreas. They should be large, dark colored, and have a large amount of lipid droplets when examined under a light microscope.  The muscle to gut ratio of the sixth abdominal segment should be greater than 4:1. The shrimp postlarvae should come from a nursery with a normal population density (100,000-150,000 nauplii/m3), not overcrowded, and the nursery should not use antibiotics. It will be beneficial to know the water temperature during the larval rearing. When the temperature is low, the shrimp larvae may eat less, become weak, and grow slowly. If it is possible to assess the quality of the shrimp larvae, the same method should be used for evaluating black tiger shrimp postlarvae, taking into account the amount of lipid and the vibrio bacteria count in the hepatopancreas.

2/ Prepare the pond and the water well

In areas where there have been outbreaks of EMS or white spot syndrome virus, it is necessary to disinfect the water. This can be done by adding chlorine (calcium hypochlorite) at the recommended concentration or by using other disinfectant chemicals. It is not recommended to use only organophosphate insecticide such as trichlorfon or cypermethrin (a synthetic pyrethroid) to kill the viral carrier organisms such as wild shrimp or crabs, because it cannot get rid the water that are tempered with disease-causing microorganisms or parasites. For water with a great deal of sediment, the water should be left to settle for some time before being pumped into the shrimp pond for treatment.

3/ Limit conditions that put the shrimp at risk of disease

3.1 The water pH should not be too low during the first month

Do not add too much probiotic bacteria or effective microorganisms to decompose organic materials at the beginning of the culture period or water preparation stage to keep the water clean in this stage. Use caution when adding effective microorganisms to reduce the water pH in order to prevent vibrio bacteria growth or to reduce the toxicity of ammonia. The pH of water that is lower than 7.5 in the morning can put the shrimp at risk. The shrimp culturalist should also take into account the rainfall. If there is heavy rain for several days, the water pH will drop. Water alkalinity will also drop, because rainwater has practically no alkalinity and the pH of rain water rainwater is 6.8-6.9. Rainfall over a long period will cause the water pH and the water alkalinity to drop. In many places, such as Rayong, Chantaburi, and Chachoengsao provinces (eastern cultivated areas in Thailand), at the times when many shrimp died from EMS, it had rained almost every day. Large numbers of shrimp died; their bodies softened, and their muscle tissue whitish. The same problem with EMS deaths following heavy rain was reported in Southern provinces of Thailand during the monsoon season from October to December 2012. When shrimp postlarvae are stocked into the pond, the water pH should be about8.0±0.2, because this is the normal seawater pH at hatcheries. The postlarvae will not have a hard time adjusting. The water alkalinity should not be below 100 mg/l. During the first month, shrimp farmers should add lime to the water to adjust the pH every time it rains heavily.

3.2 Regulate the feeding program

Following the principle of feeding generously for fast-growing breeds of shrimp in order to get them to all grow quickly to uniform size is quite risky. It is not a practice that will help prevent EMS. Shrimp culturalists do not want to see their shrimp all grow very quickly and then die quickly; they want to see them grow at a normal rate, remain healthy, and then be able to harvest them all when they reach the target size. If the amount of feed given is controlled, then the pond bottom will remain clean. There will not be an increase in vibrio bacteria that could make the shrimp sick. If there is no excess feed, then the water quality will remain good. Shrimp that are growing at a normal rate will not have to molt too often. There will be little chance of them dying with soft bodies after molting.

3.3 Carefully regulate the water quality

It is important to make sure that there are enough aerators to support the number of shrimp in each pond, because the dissolved oxygen (DO) level in the water is the most important factor for raising shrimp. If the water DO level is kept above 4 mg/liter or 4 ppm throughout the culture period, then the shrimp will grow well and remain healthy. There will be little risk of disease. It is not necessary to mention here how many aerators are needed for each size of pond, or how much horsepower is recommended for the water surface area. Shrimp culturalists who have been raising shrimp are well aware of this already. The main principle is that when all the aerators are turned on at once there will be enough water current to sweep sediment to the middle of the pond.  

There are several characteristics that were shared on most of the farms observed that did not suffer EMS deaths even when nearby farms did. These may form general guidelines for preventing EMS.  

1/ Use disinfectant from time to time

The different farms had different methods, using different kinds of disinfectant in different amounts. The main point is that the disinfectant should be one that is permitted for use in aquaculture by the Fisheries Department. Some farms might not use a chemical disinfectant but instead use the method of suspending feeding at intervals, such as once every 10 days, or reducing the amount of feed if the water color darkens. They may also add effective microorganisms to treat the pond bottom if aeration is sufficient.

2/ Add minerals or sea salt

Adding minerals or sea salt before the shrimp reach the molting stage or while they are molting can help insure that there are enough minerals in the water for the shrimp to grow new shells.

3/ Add lime to adjust water pH

Any kind of lime or calcium hydroxide will do; it is

up to each farmer’s preference. The reason for adding lime is to keep up the pH and alkalinity of the water after heavy rain. This will help guard against toxicity from hydrogen sulfide gas that forms in the middle of the pond. Hydrogen sulfide can become highly toxic when the pH drops to a low level, especially after heavy rain.  

What to do if shrimp are dying and it looks like EMS

If shrimp start to die about one month after release into the pond (it may be slightly before or after one month), their muscle tissue is opaque and whitish, and their hepatopancreas appear paler and smaller than in normal shrimp, the farmer should takes the following steps:

1/ Stop feeding immediately

You do not need to fear that if healthy shrimp eat the diseased or dying shrimp then they will catch the disease. The reason to stop feeding is so that the shrimp will not become ready to molt. The natural mechanism will cause them to postpone molting when food is not plentiful. If they do not molt, then more shrimp will not die. Once feeding is suspended, the number of new shrimp deaths should slow down and eventually stop after 3-5 days.  

2/ Add lime to raise the pH

Suspending feeding may not be enough. The farmer should add lime to the water so that the pH is up to 7.9-8.0 in the early morning before the sun comes up. This will also slow down molting and hence reduce the death rate. In ponds with low salinity or high population density, sea salt or supplementary minerals should also be added to help the shrimp recover and get stronger more quickly.

3/Fully turn on all aerators

When feeding is suspended, the condition of the pond bottom will improve, especially when the aerators are turned on full power. Then the water quality will improve. Once there are no more shrimp deaths and the shrimp in the feed trays appear normal, that is, they don’t look whitish any more, which may take 7-10 days; then the farmer can gradually begin feeding again, and should be able to continue raising the shrimp to harvest age. The overall death rate may be about 15-20%. However, if the farmer starts feeding too much too soon, there may be a second wave of shrimp deaths, and the situation will be more difficult to control than before, because the shrimp will die off quickly in the second wave. If there is a second occurrence of EMS, it is usually best to harvest the remaining shrimp immediately, because if you wait there may be a large number of dead shrimp on the bottom of the pond. 

The above information covers what is known about how to prevent EMS in Pacific white shrimp culture. Shrimp culturists may consider the information and decide which parts are practical for implementing on their own farm, and they may experiment with other methods as well. Then they can keep records to determine which practices are most effective for preventing EMS in their particular circumstances.