The success of brackish shrimp farming depends greatly on supervising and management of shrimp pond environment, especially at night. This article introduces the recommendations of Soraphat Panakorn (Aquaculture Asia Pacific, Novozymes Biologicals, Thailand) on the management of shrimp ponds with shrimp farmers so that caderes supervising the environment of shrimp farming can refer to.
At night, pond water temperature is lower than daytime . Heat slowly released from the surface of pond water causes temperature stratification that hinders oxygenation in water. Low temperature makes H2S more toxic to shrimp.
As the temperature decreases, shrimps tend to move into the muddy area, exposed to toxic gases and pathogenic bacteria. Shrimps reacting slowly to temperatures will operate less. When the temperature decreases by 1oC the metabolism of shrimps will decrease by about 10%.
To minimize the effects of temperature changes, farmers have to run aeration to prevent thermal stratification in the pond and keep the pond bottom clean with less organic by feeding properly. Avoid feeding shrimps at night because shrimps use poor food when the temperature is low.
pH in the pond changes due to two sources: ion in water and activity of phytoplankton(TVPD). At night the low operation of low TVPD causes the decrease of pH. Shrimps will soon molt when the pH reaches 8.3 or lower.
Low pH increases the toxicity of H2S. If the difference between day and night pH 1.0 will increase stress on shrimps, weaken shrimps.
To stabilize the pH in the pond, Alkalinity needs to reach 100 ppm. Farmers should regularly check Alkalinity, at least once every 3-4 days. To raise Alkalinity, at night when shrimps do not molt , lime should be added.
At night, photosynthetic activities stop, lack of oxygen occurs. Aeration to maintain oxygen is essential, do not stop aeration even if only 1 minute. The dissolved oxygen content in ponds is 4 ppm at 4 am and measured 30cm from the bottom ,3 m from the shore. Lack of oxygen causes H2S more toxic, other toxic gases are released, pathogenic bacteria develop, shrimps after molting will die, reduce the efficiency of food use.
Normally, in ponds, oxygen is made from two sources: aeration and photosynthesis. Oxygen is high during the daytime and decreases at night and is lowest at midnight. Oxygen will be low after about 1 hour of sunrise when the photosynthetic activity of the TVPD back.
When nighttime oxygen levels decrease, shrimps will reduce activity, most shrimps will lie at the bottom and those that require higher oxygen levels usually try to crawl along the pond edge. When oxygen is sufficient at night, most shrimps will swim in the pond. Oxygen should be checked at 4 am and ready to be added to oxygen as needed. Farmers should regularly check and maintain the optimal dissolved oxygen in the pond. Farmers can estimate that every 400 kg of shrimps in a pond needs 1 horse power. Shrimp volume in ponds can be calculated based on density, survival rate, shrimp weight and% feed consumption per day.
H2S at the concentration of 0.02 ppm is toxic to shrimps and many aquatic animals. It is also the lowest concentration that the farmers can smell. In that gas, NH3, NO2 is toxic only when concentrations are higher than 100 and 1000 times. H2S is present when organic matter and pond water are lacking oxygen. H2S causes more damage to fish farmers, shrimp farming than other causes.
H2S is toxic when low temperature, low pH and low oxygen. Therefore, H2S is highly toxic at night. The mild effect of H2S makes shrimps weak, easily to get infection, when severe H2S leading to the sudden deaths of shrimps. When there are other adverse conditions such as heavy rain, strong winds, lack of aeration, molting and plankton, the next morning the farmers will see the high rate of death shrimps.
In order to deal with toxic gases, pH should be maintained at about 7.8 – 8.1. Not be higher because shrimps will be poisoned by NH3. Always maintain optimum oxygen, good sludge treatment. Some farmers use microorganisms to control H2S. Ponds with high H2S make shrimps weak, night pond monitoring is very important.
At night, no photosynthesis, pH will reduce, shrimps reduce the absorption of minerals compared with daytime. When molting,shrimps absorb many minerals. When the minerals in the pond are low, TVPD have not enough minerals to operate, shrimps will die in the morning and pH test will see the pH lower than the previous day 0.3 – 0.5. It can be predicted that in the next 2 days there will be a decline in TVPD.
In case of need to supplement minerals for shrimps, supply at night. Mineral supplement for TVPD should be added in the morning. Loss of TVPD is disastrous. When the TVPD ends. pH and dissolved oxygen will drop dramatically, organic matter in the pond will increase, pathogenic microorganisms will spread, large amounts of toxic gas will release. Changes will harm shrimp farming.
In order to prevent the loss of TVPD, it is necessary to maintain a reasonable level of mineral in the pond. Regularly check calcium, magnesium, phosphate. Controlling and supplementing minerals will help maintain a stable TVPD. Beneficial microorganisms convert organic matter into inorganic substances to provide sufficient minerals and nutrients. Acting quickly and promptly when there are signs of decay of TVPD is very important, especially after the heavy rain.
The molting of shrimp is related to pH. If pH is higher than 8.3 ,shrimps will wait until the pH is lower to start to molt. pH which is suitable for shrimp molted is 7 – 8. When molting, shrimp require oxygen nearly double, after moulting about 3 – 4 hours,shrimps’s shell will become hard. If the shrimps’s shell is not hard enough after molting, soft shell, shrimps will die very quickly. When molting, shrimps need minerals to form the shell. Farmers need to pay attention to Alkalinity and minerals in ponds.
A slight decrease in meal time is a sign that shrimps are preparing to moult. We can estimate the time between two molting times of shrimps based on the measurement of the length (cm) of shrimps from telson to rostrum. The length in cm of shrimp is the number of days between two moulting times. For example, the length of shrimps is 7 cm or shrimp will molt within the next 7 – 8 days.
Farmers should record the day of shrimp’s molting and calculate the next day moulting. Farmers should also detect whether the amount of food shrimps used reduce or not , enhance night-time aeration when shrimps moult and ensure no H2S in the pond by using microbiological preparations to remove H2S. When the density of shrimps molted is to high, it is necessary to supplement minerals . Alkalinity needs to be maintained at 120ppm. In the morning after molting,farmers should observe shrimps and check the pond water quality. If the shrimps’s shells are soft or shrimps are dead, or Alkalinity sudden drop more than 20ppm or pH reduce from 0.3 to 0.5 compared to the previous day, farmers need to supplement minerals immediately.
Shrimp pond management at night
It is necessary to have experienced, responsible technicians manage shrimps in the pond overnight. The operator must check the activities of shrimps and the aeration, measure dissolved oxygen, especially when the TVDP fall, shrimps is molting, it is heavy rain and renew water. PH measurement at 9pm is also essential.
The time between midnight and dawn is very important. Problems with shrimp farming usually begin around this time, when weak shrimps float on the surface of the pond due to the effects of stress.