Environmental problems both within and beyond the farm environment pose a major constraint to achieve sustainable growth of the shrimp industry. Rains also present a significant challenge for shrimp farmers all over the world. When the rainy season arrives, shrimp production naturally declines, reflecting the management challenges that rainfall brings to a farm cycle.
Direct effects of Rain
Rainwater generally has a temperature that is 5° - 6° Celsius lower than the environment, but it can be much lower if it is associated with massive low-pressure
systems. As a result of the dissolution of carbon dioxide (CO2), rain is a weak solution of carbonic acid with a pH of 6.2 to 6.4 (in non-industrial areas). Rainwater
also lowers the temperature and pH of the shrimp ponds. Additionally, salinity and hardness also decrease due to the reduction in ion concentrations in solution.
Other physical changes directly related to rain include the increase of suspended solids due to the transport of soil material from the pond levee. The resultant higher pond turbidity negatively impacts sunlight penetration and causes abrupt crashes of the phototrophic populations. The formation of a halocline (a strong vertical border or
salinity gradient between layers of water with different salt contents) in the ponds can often be observed due to the difference in salinity between the rain and the
pond water, because the less dense rainwater floats on the saltier pond water.
Impact of rain in Shrimp farming Temperature
Environmental temperature has a profound effect on the metabolic rate of all poikilothermic organisms (coldblooded organisms, have variable body temperature
like temperature of its environment), and shrimp is no exception. Normally, a reduction in feed consumption results in around 10 percent (dry weight) for each
degree Celsius lower water temperature. Because rains can lower pond water temperature by 3° to 5° Celsius, a minimum, temporary reduction of 30 percent in feed consumption can be expected. In addition to the reduction of appetite, these thermal stratification conditions will make the shrimp migrate towards pond areas with higher temperature and salinity and possibly away from the sound of rain on
the pond surface. One consequence is a significant increase in the shrimp density in some deeper pond areas, where dissolved oxygen levels are the lowest and
H2S concentrations are the highest in the entire pond.
Impact of pH
Rainwater has a pH of 6.5 to 6.7 and shrimp ponds usually have values between 7.5 and 8.5 which leads to a drop of 0.3 to 1.5 during rains. This variation in
pH often results in abrupt die-offs of phytoplankton population in pond. It is however important to note that salinity decrease does not have the same effect, but in fact, the cyanobacteria dominate under low salinity conditions. These massive phytoplankton mortalities provide a huge number of simple sugars to the pond ecosystem as autolysis breaks down cell walls and release the cytoplasm into the water. Within a few hours there is typically an exponential increase of heterotrophic
bacteria that begins to assimilate the sugars.
Dissolved Oxygen
Dissolved oxygen (DO) level is the most critical factor in shrimp production. Its saturation level in water is 25 times lower than it is for ambient air at the same
temperature. Therefore, dissolved oxygen will always be the first limiting factor in the aerobic production of aquatic biomass. Although the decline in the pond water temperature and salinity due to rains increase the maximum capacity (saturation point) of oxygen absorption by pond water, the lack of photosynthesis will be the
determining factor regarding the DO levels in the pond. This combined with the increase in biological oxygen demand (BOD) by heterotrophic bacteria in the
absence of additional (mechanical) aeration, can lower the dissolved oxygen to dangerous levels (equal or less than 3 ppm) in less than half an hour. The condition of
black gills is one of the multiple manifestations of H2S toxicity in shrimp.
Salinity and Hardness
Both salinity and hardness are functions of dissolved ion concentrations, so if there is an increase in the pond water volume, the concentrations of all ions will decrease.
It is unusual for shrimp mortality to be directly related to salinity during the grow-out phase; however, there will be significant impact on the level of animal homeostasis (internal autoregulation to be in a stable state). The shrimp post-molting phase involves the active absorption of calcium and magnesium ions to harden their exoskeletons (shells), and this process cannot take place in the absence of these ions. Consequently, there will be a significant increase in cannibalism and mortality related to secondary infections caused by opportunistic pathogens.
Wind and Wave action
The wind acting on the surface of a body of water creates waves due to the transfer of kinetic energy from the air to the water. The amplitude and wavelength of
the waves (the factors that determine the magnitude of energy transferred) depend on the force of the wind, the duration that it blows for and the “fetch” or distance at
which the wind acts on the surface of the water. When they break, waves transfer their energy to the pond levee, resulting in erosion of the most exposed levee increasing the pond water turbidity and reducing sunlight penetration. This leads to phytoplankton dieoffs and all the problems associated with the absence of a healthy microalgae population in the pond. Another effect is subtler but equally or even more
problematic. All earthen ponds have an aerobic microlayer on their muddy bottoms, which normally acts as a barrier between the anoxic subsoil and the relatively oxygen-rich water.
Indirect effects of rain
A chain of events is often observed in the pond due to rainfall. There is almost always an abrupt drop (crash) in microalgae populations just after or during the rains.
The factors most involved in this phenomenon are the drop in pH (relative acidity of the rain), the reduction in concentration of minerals and micronutrients, the increase in turbidity and finally the reduction of solar intensity. The populations of heterotrophic bacteria – with the role to decompose organic matter – increase
exponentially due to the increase in the availability of nutrients from dead algae cells that settle on the pond bottom. This chain of events ending with low DO, pH and low temperature conditions create a very unfavorable environment for shrimp farming. Vibrio spp. typically dominate in these conditions and so do all potential pathogens- proteolytic or hemolytic or both. And under these same conditions, the sludge
oxidation/reduction will probably be negative leading to increase in hydrogen sulfide (H2S) which further escalates the diseases in shrimp- especially Vibriosis.
Better practices to overcome the impact of heavy rains:
Before the rains:
Clean and enlarge the drainage channels. Place bags of calcium carbonate (500 kg/ha) on the walls. When it rains, the calcium carbonate dissolves and penetrates through the walls, maintaining the pH and hardness at acceptable values.
Repair and compact the slopes of the levies and dykes and protect the areas of greatest erosion with sandfilled feed bags and with barriers of chopped cane
stalks. Ensure that all pond drainage gates allow for surface drainage If the shrimp farm has them, test all aeration equipment and the electrical network installation and
control panels Apply Probiotics (Skretting AquaCare Control). It regulates sludge as well as degrades organic matter and maintain good pond environment.
During the rains:
Drain surface water.
Measure DO and pH continuously, and if the pH drops,
apply calcium carbonate.
Reduce feeding by 70 percent of the normal ration and
keep reducing it according to the temperature and DO
data.
Turn on all available mechanical aerators and always
try to maintain DO levels above 4ppm.
After the rains:
Once the temperature rises, pH and DO values become normal and as the shrimp population is known, increase the amount of aquafeed in the pond progressively.
Add vitamin C, potassium, sodium and magnesium salts to the aquafeed before feeding. Maintain high levels of aeration until there is a new, stable population of microalgae in the pond. Apply Water Probiotics to control pond ecosystem.
AquaCare Control is Skretting’s water probiotic solution, which is a combination of synergetic beneficial bacteria and micronutrients on a carbon rich carrier which helps
in sludge, ammonia and pathogen control. The key benefits are improved water quality, support growth and feed conversion ratio for shrimp and fish. Apply sea salt and Dolomite to maintain osmotic balance. Apply disinfectant to ponds to effectively control pathogenic microbial loads. AquaCare 3D is Skretting’s powerful disinfectant which has a broad-spectrum action against bacteria, virus and fungi. It helps in
reducing pathogen loads under challenging farm conditions like monsoons.
Apply minerals to the pond to maintain good osmotic balance as well as mineral equilibrium. Skretting’s AquaCare Mineral Balance is a combination of macro
and trace minerals that support shrimp development. The highly bioavailable proprietary formula with optimum Ca : Mg : K ratio helps in maintaining the
ionic balance in water which support better moulting, faster growth and higher survival.
Conclusions
Rainfall disturbs the water quality parameters such as DO, pH, turbidity, and increases the microbial loads causing severe problems in pond environment. The problems include anoxia, secondary infections, cannibalism, H2S toxicity and other issues associated with incomplete molting. Excessive rainfall also leads to shrimp mortality. This mortality commonly occurs two to three days after the rains, so adopting better
management practices in farming with application of probiotic, disinfectants and minerals can control the damages caused by heavy rains.
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