Saturday, June 25, 2016

Night time in shrimp farming

Author: Soraphat Panakorn
Published in November/December 2015 AQUA Culture Asia Pacific Magazine

Consider the night activities at shrimp farms for best management practices and success with crops.

Just like any industrialised and biological business, shrimp farming involves a 24-hour work cycle from the day of stocking to harvest. While the farmer sleeps, the micro-organisms and

phytoplankton activities continue in the pond. Contrary to the adage that ignorance is bliss, unknown to the farmer a lot of happenings are taking place in the pond at night, and these
activities can have major consequences which may be disastrous the next day. The aim of this article is to show how important it is for farmers to understand the goings-on in the pond at night, followed by some guidelines for the farmers to adopt mitigation measures.

Water parameters
In the shrimp pond pH changes arise from two sources: ions in the water and activities of the phytoplankton. During the night, phytoplankton activity is low and this causes a drop in pH. In turn, ready to moult shrimp immediately start moulting once the pH reaches 8.3 or below. 

On the other hand, the lower pH will increase the H2S toxicity. Furthermore if the difference in pH between day and night readings is as high as 1.0, the stress level of the shrimp is increased and this weakens their constitution.

To stabilise water pH, alkalinity should be 100 ppm or more. The farmer must check pond alkalinity continuously, at least every 3-4 days and improve on the level with the application of lime or sodium bicarbonate at night when shrimp are not moulting.

Night water temperatures are always lower than day temperatures. As heat is released slowly from the pond surface, the differential creates temperature stratifiation which blocks the mixing of oxygen. Low temperature will also cause H2S to be more toxic to animals.

As temperatures decline, weaker shrimp tend to move to the sludge areas, exposing themselves to toxic gases and pathogenic bacteria. Shrimp also react to lower temperatures by slowing their activity. Shrimp metabolic rates decrease by 10% with each °C drop in temperature.

To minimise the impact of temperature changes, the farmer must continue to run aerators to mix water thoroughly so as to prevent water stratifiation, and to keep the pond bottom clean with less organic matter by implementing better feed management. They should also avoid feeding at night since shrimp do not feed well at lower temperatures.

IT farm in Phuket, Thailand. Long arm aerators pushing streams of water all over the
pond can buffer heavy rain effect at night. Powerful spotlights help staff to clearly
observe the shrimp activity at night

Dissolved oxygen
It is common for shrimp farms to encounter shortage of dissolved oxygen (DO) at night when photosynthesis stops. Aeration is the only way to maintain DO levels and these should not be stopped even for a single minute. The shortage of DO at night may cause other problems later on. The optimal DO concentration in the farm is 4 ppm at 4 am and the checking point is 30 cm from the pond bottom and 3 m away from the sludge edge. Insuffiient DO could lead to serious H2S toxicity, release of toxic gases, bloom of pathogenic bacteria, mortality after moulting, and reduced feed consumption. All these are stressful to shrimp.

Usually in a pond that relies on oxygenation from two sources, aeration and phytoplankton, the phytoplankton gives a typical colour to the pond water (brown to green or dark green). DO will be very high during the daylight hours and will drop slowly after night fall and reach the lowest point at about midnight. DO will remain low up to one hour after sunrise, when phytoplankton activity resumes.

With declines in DO at night, shrimp will slow down their activity in order to use less oxygen, hence most of them will touch the pond bottom and those requiring higher DO will try to

crawl along the sides of the pond. If DO is good enough at night most shrimp will be swimming all over the pond.

The recommendation is to check DO levels at 4am and provide a spare source of oxygen supply for any emergency. The farmer should frequently check and maintain pond water at optimal DO level. The farmer can also use these rough calculations: each 400 kg shrimp biomass needs 1 HP of aeration. The biomass can be calculated from stocking density, survival rate, shrimp weight and % feed consumed per day. 

H2S toxicity
A level of H2S at only 0.02 ppm is toxic to shrimp and many other aquatic animals. This is also the minimum level where farmers can smell H2S. In comparison, toxic levels of NH 3 and NO2 are 100 and 1,000 times higher. H2S is present where there is organic matter and water but no oxygen. Thus, a shrimp pond is one of the most likely places for H2S to be generated. Both fih and shrimp farmers around the world lose crops to toxic H2S more than other causes. Often they lose all their crops but as they do not know how to deal with this problem, they just accept the mortality as a common occurrence.

Conditions for toxicity of H2S are low temperature, low pH and low oxygen. Hence, night time is a good time for H2S build up, creating problems for the shrimp. A slight exposure to H2S will make shrimp weak and easily succumb to infections or diseases and a strong exposure could lead to sudden death. With additional adverse conditions such as heavy rains, strong winds, malfunctioning aerators, moulting and plankton crash the farmer will experience high shrimp mortality the next morning.

To deal with this toxic gas, the farmer must maintain a stable pH at around 7.8-8.1. It cannot be higher than this as the shrimp will be poisoned by NH3. Farmers must maintain optimal DO levels always and manage sludge well. Some farmers use some H2S oxidising bacteria to control H2S. Many farms with high loads of H2S will show weak shrimp at night. Thus, night observations are crucial- (For readings of un-ionised H2S at different pH and temperatures,refer to Boyd, 1990, Water Quality, an Introduction, p217).


During the night, with no release of oxygen from phytoplankton, pH will decrease and there will be lower uptake of minerals compared to during daytime. For moulting, shrimp will use up a high volume of minerals. When minerals in the ponds are limited or low, phytoplankton will not have enough mineral supplies the next day, when the sun rises. In such situations, plankton will start dying and on checking the pH, the farmer will discover that it was lower than the previous day by about 0.3-0.5. In this case, the farmer can predict that within two days, there will be a massive plankton crash. The farmer must get ready to manage the situation properly.

In the event that a farmer needs to apply minerals for the shrimp, the application must be done at night. If the minerals are required for the plankton, these should be applied during late morning. Plankton crash is a shrimp farmer’s worst nightmare. When a crash occurs, pH and DO levels will suddenly drop. Organic matter in the pond will increase and micro organism populations will bloom within a short time. Large amounts of toxic gases will be released. All these factors seriously affect shrimp health.

To prevent a plankton crash, we need to have the right mineral ratio. It is necessary to frequently check for calcium, magnesium, potassium and other elements. Monitoring and adding enough minerals will help maintain stable bloom. Benefiial microorganisms that can turn organic matter to inorganic matter could help provide sufficient mineral and nutrition to stabilise phytoplankton bloom. Quick action to respond to any crash is important, especially after heavy rains.

Shrimp behaviour

Shrimp can moult any time but this is linked to pH. If pH is over 8.3, shrimp will wait until this is lower to start moulting. The ideal pH for moulting is 7-8. This is why shrimp in a pond with
phytoplankton colour indicating that pH is a bit higher than 8.3 during the day, will moult at night. When shrimp moult, they will require the oxygen in about double the time. They need about 3-4 hours for the shells to harden. If they cannot accomplish this and the shell is still soft, shrimp will soon die. During moulting, shrimp need to absorb minerals for new shell formation from the water. The farmer then needs to pay attention to alkalinity and minerals at this time.

A drop in feed intake for the afternoon meal, is a sign of moulting. As a guide, we can roughly estimate the interval by measuring shrimp length from the end of the telson to the

rostrum in cm. The length of the shrimp in cm indicates the time interval in days between one moulting to the next. For example, if the length of the shrimp is 7 cm, this shrimp will moult again in another 7-8 days.

In this case, the farmer must record the moulting date, and calculate the time for the next moulting. The farmer must also detect the drop in feed, run spare aerators for the night of

moulting and ensure the pond is free from H2S by applying H2S oxidising bacteria. The farmer should also apply minerals when shrimp moult in the case of high stocking density or low salinity water or there was already some mineral shortage during the last moult. The alkalinity should be kept at 120 ppm. In the morning after the moult, the farmer must observe shrimp and check water parameters. If some soft or thin shell shrimp or dead shrimp are discovered, or there is a sudden drop of alkalinity of more than 20 ppm or phytoplankton drop (pH drop about 0.3-0.5) from the day before, the farmer needs to apply minerals immediately and also during the next moulting.


Shrimp exhibit predator behaviour at night. Shrimp will break from the crowd as they prefer to search for feed independently rather than wait for the feed supply. Feed should be reduced for the night so that shrimp will on feed left over feed from previous feedings. In
this way, the farmer gets optimal feed consumption

Farm staff
In a farm, keeping awake at night is not easy, especially with the sounds of aerators humming like lullabies, and after a full day of work. Even staff assigned to night duty might fid it diffiult to keep awake. A farm owner must assign the most reliable, honest and capable person to be a shrimp guardian at night. The person should have all the equipment such as vehicles, radio or mobile phones, light set or spotlight torches with spare batteries, repair tools, weapons to protect themselves and properties, and products duty to be used in an emergency. This person must be free of work during the day, must have enough knowledge on shrimp farming and should be paid special hardship wages.

The night guardian must check shrimp behaviour by observing shrimp in check trays or use a torch to observe shrimp activityHe or she should observe the colour of the light reflction of the eyes of weak shrimp which are paler than those of strong shrimp. However, when using a torch on a pond, the guardian must move the torch slowly. Fast moving light over the water could make shrimp nervous and jumpy, thus increasing stress and weakening

the shrimp.

The job of the night guardian is also to check that all aerators are functioning well. The DO meter must be used frequently and randomly to measure DO at night especially when phytoplankton population drops, shrimp is moulting and when there is heavy rain and new water exchange. Checking pH after 21.00 h is also part of the job. A night guardian can gain suffiient experience to undertake the job well after 2-3 months

Other problems
Farm security

A watch tower with high power spotlight and light colour fencing would help to keep would be intruders away. One effective deterrent is to have fencing under the water at the pond side with barb wire to trap illegal cast nets. Another way to prevent intruders is to have farm staff housing around the farm border.

The likelihood of accidents happening is highest at night, especially during bad weather such as heavy rains. To be safe, repair work, must be carried out with two staff working together. Each farm must have a safety protocol and a safety exercise at least twice a year. It is important to check that all equipment are working well.

Farms must have enough equipment for night work such as safety clothes and shoes, helmets, eye glasses, umbrellas, fist aid medical sets, spotlights with long electric cords and head lamps. etc. This is to allow staff to work comfortably at night. Most accidents occur at night because of unsafe conditions. It is also important that all in the farm know where to get and how to use equipment.

Aerators not running

Since oxygen is crucial at night, malfunctioning of aerators can cause huge losses. Thus repair tools, electric generator sets or spare engines to run paddle wheels in cases of electricity cut offs, must be properly maintained and regularly tested. Farm staff with basic mechanical training is important. At least two or three farm staff should have some training on how to fix general problems with equipment failure. As an encouragement, this group should be remunerated for their skills.

Midnight to sunrise

An important time to keep watch is from midnight to sunrise. Problems with shrimp in the ponds can usually be first detected during this period before the full-scale problem is realised a couple of days later. The weakest shrimp (but still in a good condition) will surface and the farmer can see them. They are surfacing because of some stressful condition.

Heavy rain

Once this occurs, especially at night, there should be immediate action to prevent stress to shrimp. Heavy precipitation such as 30 mm of rainfall will worsen the generally night time bad conditions for water parameters such as DO, pH, temperature and H2S toxicity. Together with salinity drop, this will cause a phytoplankton crash and low feed consumption the next morning.

Fresh water should be thoroughly mixed, DO and pH should be monitored and stabilised, proper aeration should be provided and lime applied. Applying burnt lime around the sludge area to block shrimp from sinking down to the sludge area will help. Farmers usually do nothing during a rainy night and later, within a week, find some mortality which slowly increases. They fail to take early action and finally end with losses and are forced to harvest. Proactive monitoring at the beginning is most helpful

The message

These events happening at night are often not ‘noticeable by eye’ and the neglect is perpetuated when farmers and their staff have the idea that ‘it could not be any large problem and we can wait to observe it the next day’. However, with farming conditions such as soil, water, climate and post larvae quality, weakening more than before, many farms experience problems occurring at night. Thus, real time and on time detection and quick handling and management will bring success. If a farmer cannot find a good night guardian, he could play safe by stocking below pond carrying capacity. However, even with this, more attention is needed during nights with heavy rain.

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