How does rainfall affect shrimp pond water parameters?
Problems in shrimp farming usually occur during severe drought or heavy rainfall. Observations confim major losses occur mainly during periods of rainfall.
In the last two years, we have had droughts brought about by El Niño. The US Climate Prediction Center and the International Research Institute for Climate and Society have issued an announcement that there is a 60% chance of La Niña developing in the last quarter of 2016. In contrast to El Niño, with La Niña we will witness prolonged storms, heavy rainfall, heavier monsoons and severe winds resulting in more hurricanes and tropical storms. What does it mean and how will it affect shrimp culture in South East Asia?
The most dangerous phenomenon that can affect shrimp culture can be attributed to rain. In general when farmers recalled when they encountered problems, it was more often during the monsoon seasons. Major losses occur mainly during the rainy season. In 2016, we had hot weather for the fist 6-7 months and now the rainy season is coming. The La Niña effect can usually last for 2-3 years. How will excessive rainfall affect shrimp pond water parameters?
Rain has very serious effects on shrimp culture. Effects on shrimp include cramping, loss of appetite and reduced feed consumption, shrimp parking at the side of bunds (2-3 days after the rains) and black gills or dirty shrimp. From some observations and records, in south Thailand, shrimp mortality may range from only 2-3% to 50%. We gathered statistics and co-related the data to weather reports. There is no doubt that heavy rainfall can cause huge mortalities. However, the signs of impending mortality are usually minimal unless the farmer knows what to look for.
It is important that the farmer is aware of several direct and indirect effects of rain on water parameters in shrimp ponds. In this article, we discuss various direct and indirect effects of severe rainfall on shrimp culture (in no particular order) and elaborate in detail key points on how rainfall affects shrimp culture.
Direct and indirect effects
The direct effects on pond water are reduced temperature, oxygen, pH, alkalinity and salinity. Sound and wave disturbances increase, and rainwater flws from the bund into the pond.
The indirect effects are phytoplankton crash and organic material accumulation at pond bottom. There will be a sudden bloom in bacterial population after the water temperature returns back to normal. The agitation of the sludge layer exposes the anaerobic layer (black soil) and shrimp will be exposed to toxic hydrogen sulphide (H2S) gases
The results of these direct and indirect effects of rainfall are:
• Once all the above occur, oxygen is depleted and toxic gases such as H2S are released
• Recently moulted shrimp, which are weak become exposed to toxic gases and pathogens and succumb to infections
• Mortality may occur 2-3 days after a severe rainfall.
During severe or prolonged rainfall and cloudy days, there will be less sunlight reaching the pond surface. Wind blowing across the surface of the pond can cause pond water temperature to drop by 2-3°C. The optimal pond water temperature should range between 30-31°C. When temperatures drop 1°C, feed uptake by shrimp typically drops 5-10%. Thus, a drop of 3°C can cause feed uptake to drop up to 30%. When water temperatures drop, feed becomes less palatable and shrimp being cold blooded are affected by external water temperatures.
Shrimp activity also slows down. They will move less and tend to gather at the pond bottom. This will drastically increase shrimp density at the pond bottom. When this happens, being naturally competitive, shrimp will experience more stress as they compete for limited oxygen and space.
As the water surface is cooler following the rains, shrimp will move towards the warmer areas in the pond which unfortunately is usually the sludge area. Here the shrimp become exposed to toxic H2S gas and pathogenic bacteria. In these areas, oxygen levels are normally low, but during a rainy period, oxygen levels may drop to zero.
During normal temperature fluctuations, microbial activity increases with increased temperature which accordingly reduces the organic load. Once there is a sudden drop in temperature, microbes also reduce their activity. This leads to the accumulation of more organic material in the pond. When temperatures rise again after a few days, there will be a sudden massive bacterial bloom as 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.
Reduction in immunity
Heavy rainfall can cause pond water pH which usually is around pH 8, to drop. The pH of rain is usually around pH 6.5-7.0. Rain will directly drop pH by 0.3-1.5 in a very short period of time. This causes an immediate decrease in phytoplankton activity.
When pH drops, this causes the toxicity of H2S to increase. H2S is highly toxic at low pH. Shrimp will also be stimulated to moult under adverse conditions of low oxygen, increased density on pond bottom, increased H2S toxicity, low salinity and alkalinity. All these conditions combined increase the chances of moulted shrimp dying within 2-3 days after heavy rainfall. However, often this level of mortality is not noticed because the soft shelled dead shrimp are eaten by other shrimp.
Indirectly, farmers will only notice this occurrence when the average daily growth (ADG) is not improving. What is the key sign of this condition? When the feed uptake drops for 1-2 days after heavy rain, cannibalism occurs. Finally, the effect of the sudden pH shock results in lowered shrimp immunity.
Low dissolved oxygen
In a pond, there are usually two sources of dissolved oxygen (DO): from the aerators and from phytoplankton. During prolonged rainfall, plankton activity will slow down as there is
less sunlight available. This is undesirable; even though shrimp activity decreases due to the changes in temperature, its oxygen requirement is still high or as per normal. DO is supplied by aerators and if the water is not mixed properly, pond water stratifiation/stagnation will occur. The layer of freshwater (stratification) on the surface of the pond makes it difficult for oxygen to dissolve into the rest of the water body. DO levels can drop from 4 ppm to 2 ppm and then to 1.5 ppm in half an hour if action is not taken immediately.
Salinity and alkalinity
With dilution of pond water with rainwater, both salinity and alkalinity levels drop. In order for shrimp to harden their shell, it needs sufficient minerals (alkalinity) in the water to do so. When salinity drops very quickly, the moulted shrimp will not harden their shell in the usual amount of time. Cannibalism will occur leading to infection of the weakened shrimp.
The plankton population will also drop due to the low light intensity, low salinity and low pH. These changes impact on the microbial population in the pond; benefiial bacteria tend to die
off allowing pathogenic bacteria to flourish. Also once alkalinity drops, pH will start to fluctuate when the buffering capacity in the pond is reduced.
The sound of raindrops tapping onto the surface of the water seems loud to us; imagine how deafening it is to the shrimp in the pond as water tends to amplify sounds. This causes a lot of stress to the shrimp. The shrimp will try to hide from the loud noises and retreat to the pond bottom. They are then exposed to low oxygen conditions, high densities, toxic gases and cold temperatures.
Waves caused by wind action
The sludge layer is covered by a thin oxygenated grey layer. Strong winds create waves which disturb this grey layer. This then exposes the anaerobic black sludge which releases various toxic gases such as H2S, ammonia, nitrite and methane. Water running off from the bund into the pond and flowing down to the pond bottom will also disturb areas with sludge accumulation and release toxic gases. On exposure to these toxic gases, shrimp become weaker and are prone to infections and diseases.
When temperature, light, salinity, pH and alkalinity change suddenly, plankton activity will reduce and may lead to a plankton crash usually within two days. The thicker the bloom, the faster the plankton will crash. We can observe this via changes in water colour and pH, or when the afternoon pH is lower or the same as the morning pH. This means that plankton is crashing or is in the process of crashing, even if the water is still green in colour. Dead plankton is still green and may still provide some green colour in the pond.
Dead phytoplankton brings about low DO as there is no oxygen production and oxygen is used for bacterial action. This means that if DO in the afternoon is 6-7 ppm it may drop to only 2-3 ppm. When the dead phytoplankton of which 90% will accumulate at the pond bottom starts to decompose, they will start to compete for oxygen. If the farmer sees cloudy water, foam at the pond surface, bubbles with a long trail and plankton flocculating, this means that the plankton has crashed.
Rain causes pH to drop drastically, as the pH of rainwater is usually lower than the pond water; slowing of plankton activity also causes the pH to drop. Under such conditions moulting occurs. When moulting, shrimp need more space, twice as much oxygen and increased mineral levels. However, conditions during the rains are not favourable for moulting; moulted shrimp have soft shell, and succumb easily to infection and mortality. Moreover, cannibalism of dead shrimp makes it difficult for the farmer to detect the dead shrimp. What we will observe is a reduction in feed and lower ADG.
Accumulation of organic matter
During rainy periods, shrimp will not feed normally, but the farmer continues to feed the usual amount of feed thus leading to overfeeding. Phytoplankton activity will also drop as discussed previously. Bacterial action will also slow down, allowing organic matter to accumulate on the pond bottom. This means that a time bomb is ticking as the bacterial population will suddenly bloom when the temperature rises, as there is excess organic load in the pond. Usually the pathogenic bacteria will be the ones, which bloom because they are usually hardier and can withstand harsher conditions. They also tend to grow much faster than beneficial bacteria.
Why is there a tendency to overfeed? This is because farmers tend to check feed in the check tray only. Feed in the check tray is easily accessible by bigger and stronger shrimp. Normally, bigger shrimp are always being surrounded by other shrimp and the former are not allowed to feed peacefully in the check tray. However, during the rainy period when most of the shrimp do not have good appetite, these bigger shrimp now have an opportunity to eat and finish off all the feed in the check tray. As the shrimp gut is fairly short and open, they are quite capable of eating non-stop until the feed is finished. This will give a false picture of the current feeding habits of the shrimp in the pond.Farmers will think that they need to increase feed whereas the opposite is true. The bigger shrimp will also 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 will lead to the accumulation of excess organic matter in the pond.
The overall effect of excessive rainfall is shrimp mortality due to H2S poisoning, soft shell issues and accumulation of organic matter. The farmer needs to understand the big picture of how rainfall can affect the various parameters in a pond.
Recommended best practices
Ideally, farmers should use available technology and predictive weather forecasts. Some weather forecast websites are weather.com and accuweather.com. If they learn to predict the weather in the next few days of culture, they will be better prepared for any eventualities. If there is going to be rain in the next two days, pond preparation should include the application of Pond Dtox (Novozymes, USA)- a bacteria product that can neutralise hydrogen sulphide, a day before or just before it starts raining.
Further to this, the following measures are recommended.
• Always make sure that the oxygen levels are 20% more than required. Switch on the aerators. All aerators should be running when it is raining.
• If there is heavy rain, allow the excess rain water to overflow from the surface.
• Assign a worker to apply lime on the bund as a usual practice during good weather. Then when it rains, the lime will leach into the water helping to maintain alkalinity.
• Assign a worker to check the water pH during rainfall. If the pH falls, apply lime.
• Stop feeding during rainy conditions.
• Mix Vitamin C and salt (minerals) with feed before or after the rains. The dosage is 5 g/kg feed. Dilute 17x water (add 5 g of salt to 80 mL of water), mix into feed, allow to air dry, then feed the shrimp. This will allow the shrimp to obtain minerals from the feed if there is a drop in alkalinity in the water.
• Once the rain stops, it is recommended to apply a double dosage of Pond Plus (Novozymes, USA) to allow benefiial bacteria to first colonise and then competitively exclude the pathogenic bacteria.
In summary, our advice to farmers is to be prepared during rainy periods as there are so many factors contributing to problems in the shrimp pond. Being aware and being prepared are the first steps to overcome the problems encountered during the rainy period. There are signs to watch out for and we need to read the signs correctly and take preventive and proactive action to minimise and prevent losses.
Source: http://www.aqtinfo.com/2017/02/how-does-rainfall-affect-shrimp-pond.html?m=1 – AQUA Culture AP Nov-Dec 2016