Rain presents a significant challenge for shrimp farmers all over the world. Rainfall can quickly alter pond conditions and trigger a chain of events that could result in sub-optimal water quality and stunted shrimp growth. Rain generally has a temperature of 5 to 6 degrees-C lower than the environment, but it can be much lower if it is associated with massive low-pressure systems. Shrimp is a cold-blooded (or poikilothermic) species. Its body temperature can only change in a limited range. But, if environmental factors change dramatically and exceed permitted limits, shrimp will be weak, shocked, and possibly dead.

When it rains, temperature, salinity, pH and dissolved oxygen in ponds will be reduced significantly. Thus, shrimp care during rainy season with continuous and prolonged rains becomes difficult for people, even with experienced farmers. As a result of the dissolution of carbon dioxide (CO₂), rain is actually a weak solution of carbonic acid with a pH of 6.2 to 6.4. These two physical factors tend to lower the temperature and pH of the shrimp ponds. Additionally, as a consequence of dilution, salinity and hardness also decrease due to the reduction in ion concentrations in solution.

Effect of rainfall on water quality parameters

Practices to minimize the impact of heavy rains:

Before the rains:

• Clean and enlarge the drainage channels. In some cases, it may be necessary to install a pump station at one end of the drainage channel to mechanically discharge rainwater when the level of the river exceeds the level of drainage.
• Place bags of calcium carbonate (500 kg/ha) on the walls. When it rains, the calcium carbonate dissolves and penetrates through the walls, helping maintain the pH and hardness within acceptable pond values.
• Repair and compact the slopes of the levies and dykes, and protect the areas of greatest erosion with sand-filled feed bags.
• Ensure that all pond drainage gates allow for surface drainage.

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 try to maintain DO levels above 4ppm at all times.
• Monitor the health of the pond microalgae by observing samples using a microscope – dead cells can still be green. Healthy algae cells have a full central vacuole and there is no separation between the cell wall and the membrane. If an algae die-off is about to happen, sometimes it can be prevented by pond water exchange to reduce the algae cell density and by raising the pH.

After the rains:

• Check shrimp activity and water environment after the rain. Check the appearance, color, reflectivity; check shrimp gut and shrimp feed in the farm, etc.
• Apply progressively increasing amounts of aquafeed in the pond as the temperature rises, as long as the pH and DO values ​​are acceptable and the shrimp population is known. It is very important to reconfirm the shrimp population estimate after the rain event. Because shrimp mortality tends to be chronic, daily population sampling should be carried out for at least a week afterwards.
• Add vitamin C, and 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.

Farming protocols for the rainy season:

• There is significant reduction in feed consumption during rainy weather. After a spell of heavy rain, it is suggested to reduce feed by 40-50% and subsequently feed strictly as per check tray consumption. Avoid overfeeding and feed wastage.
• Rain water generally accumulates as a surface layer in the pond and it takes a few hours for this water to mix with saline water below. Such situations can create bloom crash and drastic fall in dissolved oxygen (DO) levels. It is therefore very important to keep aerators running during rains. It will prevent salinity stratification of water and gradually balance salinities.
• Water temperature and shrimp feeding also has a strong co-relation. Adjust feeding as per the water temperature.
• Below 22°C – provide 50-70% of daily ration
• Between 25 to 28°C - normal feeding
• Above 30°C – 60-70 % of daily ration