Typhoons and Heavy Rain — Losses Come the Day After

Mortality is found far more often on the morning after heavy rain than on the day itself. The rain does not kill the shrimp — the layer the rain creates cuts off oxygen overnight. ■ Rainwater floats, it does not mix Rainwater carries no salt, so it is light. It does not blend in; it forms a thin freshwater layer at the surface. Mixing on its own takes hours. Once that layer forms the pond splits. The surface has oxygen while the bottom's supply is cut off — and the bottom is where the shrimp are. Add an overcast sky, which weakens photosynthesis, and bottom oxygen hits the floor overnight. ■ Salinity — within 5 ppt a day White shrimp live across 5 to 35 ppt. What matters is not the absolute value but the rate of change. A swing beyond 5 ppt in one day is more than freshly moulted shrimp can take. Heavy rain is the classic event that crosses that line in a single go. Measure salinity again after the rain stops, and compare it against the reading from before. ■ Before the rain — lower things in advance If you saw the forecast, cut the feed rate ahead of time. Shrimp eat less while it rains, and what is left decomposes on the bottom and burns oxygen. Check alkalinity too. Holding 100 to 150 mg/L keeps pH from swinging much when rainwater arrives. Take heavy rain on low alkalinity and pH drops sharply, which is a burden by itself. ■ During the rain — do not stop the aerators The most common mistake is switching aerators off while it rains. Do the opposite. Aerators do not only add oxygen. They mix, which stops the layer from forming in the first place. Mixing before stratification sets is far easier than breaking it afterwards. Keep them running through the rain and for a while after it stops. ■ After the rain — drain from the surface The fresh water is on top. So when you drain, take it from above, not below. Surface draining sends the freshwater layer out first and salinity recovers quickly. Drain from the bottom and only the saltier water leaves while the fresh layer stays. Knowing where your outlet sits is half of heavy-rain preparation. And watch the colour. Blooms often crash all at once after heavy rain. If the water suddenly cleared, that night is the dangerous one. ■ That night and the following dawn Do not skip the pre-dawn measurement on a day with heavy rain. A layer formed, the bloom was shaken, and feed is sitting on the bottom. Someone needs to be at the pond between 4 and 6 in the morning, when oxygen bottoms out. If shrimp are crowding near the surface or the pond edge, they are already short of oxygen. Run aerators at maximum and stop feeding at once. ■ Records prepare you for the next typhoon Write down the date, the rainfall, and the salinity, pH, alkalinity and pre-dawn oxygen on either side of it. How hard heavy rain hits your pond, and how many days recovery takes, differs from farm to farm. Knowing that range changes how you read the next forecast. Shrimp365 logs salinity, pH and dissolved oxygen per tank and alerts you the moment a value leaves its range. Free to use.