Unfortunately, a common conventional practice during pond preparation is the pre-stocking fertilisation of water. This practice inadvertently encourages the opportunistic and usually harmful bacteria to grow and dominate the system, resulting in the pond or tank water being densely populated with lots of harmful bacteria even before we stock the post larvae.
STRONG ENOUGH TO FACE EVERYTHING!
This is all too true and we speculated that perhaps early fertilisation is the main cause of EMS! The harmful bacteria, predominantly the Vibrios are encouraged to dominate the pond before we even stock our post larvae and so EMS strikes as early as 15 days after stocking. For this particular cycle in our R&D farm, we threw away all the conventional wisdom of adding fertilizer and fermented organic matter. Two days after disinfection of the pond water, we just added a blue colorant (for shading the pond in lieu of phytoplanktons) and probiotics to encourage beneficial bacteria growth. Seven days after disinfection, we stocked the post larvae. Our shrimp grew to 3g in 30 days without the conventional fertilisation.
Feed management program Realising the importance of low organic load in the intensive shrimp pond ecosystem, we began a stringent feed management program. We also made sure that there was no overtly strong water current to sweep away the feed in the feed tray by minimizing paddlewheels in front of the feed trays. This enabled us to observe the feed tray for remnant feed. We installed a current breaker upstream of the trays. This was necessary so that feed tray monitoring was not biased.
Basic biosecurity Biosecurity is the prevention of entry of pathogens into the culture ponds. When the post larvae were about 8 days old in the hatchery, we sent samples taken from the hatchery for PCR (polymerase chain reaction) screening for EMS, white spot syndrome virus (WSSV) and infectious myonecrosis virus (IMNV) diseases. All post larvae to be accepted must be free of all the above diseases. Visitors to the ponds were strongly discouraged. We practised alcohol based hand wash and foot bath at each feeding bridge. Vehicles were not allowed to enter the pond compound to prevent disease vectors that might adhere to tyres from spreading to the ponds. To prevent wild crabs from the outlet canal from crawling into the pond at night, we surrounded each pond with a 30cm high plastic fencing, buried 3cm into the ground.
Finally, free of AHPND pathogens In early November, after repeated rainy days, there was mortality in some ponds. Samples were sent to a private laboratory (Lab Ind Resources) for PCR screening of the AHPND pathogens. It was found that WSSV was detected but not the strain of Vibrio parahaemolyticus, responsible for AHPND.
Harvest results As the area is disease prone, we stocked only at about 70 PL/m2. The harvest results (Table 1) showed that we were able to overcome EMS for this crop. Overall, we obtained a survival rate of 58.7% and feed conversion ratio (FCR) of 1.83. The shrimp were harvested at the sizes of 10.6 to 14.1g over 99 to 109 days of culture. In this crop, the diseases affecting the ponds were WSSV and white faeces, and there was no EMS infection. In summary, we are convinced that together, these three measures helped us to alleviate EMS in shrimp farming: a central sump to regularly pump out the sludge, no fertilisation of the pond water and a strict feed management program.
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March/April 2018 AQUA Culture Asia Pacific Magazine
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