White Spot
White spot is one of the most devastating diseases to Thai shrimp farms and requires good practice to control attacks, especially in the winter time.
White spot is often mistaken as Taura syndrome as an orangish discolouration is seen. The result is that the disease is often picked up too late, when the white spots appear a few days later.
In order to prevent white spot, Dr Chalor suggests that all farmers should nurse post-larvae at 32oc continuously for seven consecutive days.
He also stated that stocking in Southern Thailand should also be done during the monsoon season and in the central parts of Thailand, stocking during the winter should also be avoided.
Brood stock should also undergo Nested PCR and a strict biosecurity practices should be implemented.
EMS
With EMS, moralities can occur in the first seven to 30 days post-stocking. Early signs of the disease include soft shell and white muscle.
First reported in Southern China in 2009, the disease has spread to Viet Nam in 2010, Malaysia in 2011 and Eastern Thailand in 2012. The disease is now found in Southern areas of Thailand also.
It is still unknown what exactly causes EMS, but Dr Chalor believes it could be due to a bacteria or a combination of many diseases. He also noted that it seems to appear differently in different areas.
There have also been links of the disease to unhealthy post-larvae from an unhealthy broodstock and several organisms being found in the hepatopancreas.
Risk Factors for EMS
Dr Chalor noted that inconsistent phytoplankton blooms during the first 30 days and a low pH of 7.4-7.6 in the morning, are all risks involved in the presence of EMS. The low pH and lots of rain have been linked to the possible cause of soft shell as the shrimps are not getting enough minerals.
By having healthy post-larvae and good management, farmers should achieve good production. With good management but unhealthy post-larvae, farmers should expect only moderate production. But, if farmers have unhealthy post-larvae and improper management, that is when EMS will most likely occur.
Improper Management
Improper management often includes overfeeding during the first 30 days. Low pH and alkalinity (due to rain), also increase the chances of EMS. Inconsistent plankton blooms and a clear water colour should also be avoided.
How to Solve
One of the most important things to manage, it the health of post-larvae. Post-larvae should not be reared in more than 100,000 nauplii/ 1 cubic meter.
Water should be well managed at 30oc and good quality feed should be used. During the first 40 days, the water should also be a greenish/brown colour.
At day 14, body weight should be not more than 1000 PLs/1g. At day 16 it should be no more than 700 PLs/1g and no more than 300 PLs/1g at day 18.
The hepatopancreas should also be large and dark, not a white colour.
Managament if EMS Found
If EMS occurs on the farm, and some shrimp die, feeding should be stopped immediately until no more mortality is seen and shrimp in the feed-tray look healthy. After seven to 10 days, gradual feeding should begin.
Disinfectants should be used to reduce pathogenic bacteria periodically and aerators should be turned on fully to provide lots of oxygen.
Minerals and salts can be added to the water to increase the levels of many major minerals, especially after heavy rain. This will provide enough minerals for shrimp that are moulting, preventing them from dying. Lime can also be put into the water to maintain pH levels.
Dr Chalor stated that the quicker farmers react, the more shrimp can be saved. If farmers react timely, around 50 per cent can be saved.
Conclusion
By controlling pH, alkalinity, mineral levels and phytoplankton blooms effectively, EMS should be prevented.
pH should be kept between 7.8 and 8.3 and alkalinity should be between 120-150 mg/l.
Sodium hydroxide can be slowly added to the water if pH very low and acidic, and magnesium can be added to maintain phytoplankton.
