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  What Happens in Shrimp Ponds During Heavy Rains?  

• Water temperatures, oxygen, pH, alkalinity and salinity drop.

• Phytoplankton crashes occur.

• Organic material accumulates on pond bottoms.

• Strong winds stir up the bottom sludge.

Toxic levels of hydrogen sulfide (H2S) are released.

• Pathogenic bacteria replace beneficial bacteria.

• The noise of rain pounding on the surface stresses the shrimp.

• Shrimp molt because of the lower pH and phytoplankton changes.

 

Outcomes:

• Mortalities occur because of the changes in water quality, stress and pathogens.

• Feed consumption drops.

• Shrimp migrate to the bottom of the ponds, where temperatures are warmer and dangerous sludge levels are higher.

• The noise of the pounding rain causes stress, which the shrimp avoid by heading to the bottom of the pond.

• Bottom conditions deteriorate because the sludge is stirred up.

• Competition for oxygen and territory increase stress levels.

• Soft shells develop because mineral levels drop.

• When temperatures drop 1°C, feed uptake by shrimp typically drops 5 to 10%.

• A drop of 3°C can cause feed uptake to drop as much as 30%.

• Shrimp become susceptible to infection and cannibalism.

• When temperatures rise again, there will be a sudden massive bacterial bloom because there is a lot of organic material for the microorganisms to feed on.  This will also take up more oxygen as the organic material is degraded in an already low oxygen situation.

• In southern Thailand, shrimp mortality may range from 2-3% to 50% during periods of heavy rainfall.

Actions:

• Turn on all aerators when it rains.

• Keep oxygen levels 20% higher than normal.

• Allow excess rainwater to flow off the surface of the pond.

• Check the pH during rainfall.  If it falls, apply lime to the top of the banks.

• Stop feeding during rainy conditions.

• Add vitamin C, salt and potassium to the feed after the rains.

Comments:

The signs of impending mortality are usually minimal unless the farmer monitors the situation carefully.  Effects on shrimp include cramping, loss of appetite and reduced feed consumption.  Changes in water color, lower pH, reduced feeding and lower growth rates indicate that a plankton crash is occurring, even if the water is still green.  Cloudy water, foam on the pond surface, bubbles and plankton flocculating also indicate a plankton crash.  Cannibalism of dead shrimp makes it difficult for the farmer to detect mortalities.

Shrimp farmers have a tendency to overfeed during rainy periods because feeding trays may indicate that more feed is being consumed.  During rainy periods, bigger shrimp have an opportunity to eat and finish off all the feed in the check tray.  This gives a false picture of the current feeding habits of the shrimp in the pond.  Farmers think that they need to increase feed whereas the opposite is true.  The bigger shrimp tend to eat more and faster because of the strong smell of the feed in the check tray, but the rest of the feed that is spread out in the pond will not smell as strong and may not be as attractive to the shrimp.  This leads to the accumulation of excess organic matter in the pond.

A Case Study: During the rainy season, at 118 days of culture, a Penaeus monodon farmer in Malaysia noticed problems in a 0.5-hectare pond, stocked at 28 postlarvae per square meter.  The pond had a heavy plankton bloom (transparency of less than 10 centimeters), and the shrimp were surfacing and showing signs of cramping.  In general, shrimp were not very active and were dull in color.  They had soft shells and loose meat.  Feed consumption dropped to 20 kg/day from 40 kg/day.
 
Vitamin C and salt were added to the feed (5 grams per kilogram) and three kilograms of potassium were added to the pond water every day.

The farmer prolonged growout to 173 days and harvested 38-40-count shrimp per kilogram.  The shrimp looked healthy and had bright colors and no longer had soft shells or loose meat.  Total production was 7.2 metric tons per hectare and the survival rate was about 50%.  Water quality had improved during the last part of the culture period.

Source: AQUA Culture AsiaPacific (Editor/Publisher, Zuridah Merican, email zuridah@aquaasiapac.com).  Shrimp Culture: How Does Rainfall Affect Shrimp Pond Water Parameters?  Soraphat Panakorn (January161975@gmail.com) and Erin Tan (erin@syndelasia.com).  Volume 12, Number 6, Page 8, November/December 2016.

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