Khí độc H2S – “sát thủ giấu mặt” của tôm nuôi

During the rearing process, the waste is collected by the fan in the middle of the pond bottom (for a muddy bottom pond, some waste is still distributed around the bottom). This waste pile is divided into 2 layers. The very thin outer layer (about 5 mm) is oxidized, so it has a relatively light color, which has the function of covering and limiting the release of toxic gases. The lower layer is black, the waste is in anoxic conditions, so the desulfurization bacteria produce toxic gas H2S. H2S toxic gas has the smell of rotten eggs, the more black mud, the more toxic H2S. It is called “killer” because the toxicity of H2S is many times higher than that of NH3 and NO2, just 0.01 ppm can kill shrimp. Still “hidden” because so far, there is no instrument to test the toxic gas content of H2S in the pond. The following is a description of developments, signs and specific measures of some cases of shrimp death due to toxic H2S gas.
1. Died after the first month of rearing (25-45 days of rearing)
Developments: After the renovation is completed, supply new water into the pond (especially for old bottom soil ponds, ponds in mangrove areas with many dead parrots and tarpaulin ponds through many farming seasons), organic matter on the bottom of the pond. dissolved in water, consuming a lot of oxygen and producing a relatively high amount of toxic gas. For black tiger shrimp, which often live in the pond bottom, this is the cause of shrimp stress (shock) and weakness, susceptible to Vibrio infection, or more seriously, shrimp will die due to toxic gas H2S (human) This is called first month rearing syndrome). For vannamei this is less severe because most of the shrimp’s activity takes place in the aquifer, but shrimp can also be weak and susceptible to disease. If shrimp are molting or rummaging for food on the bottom, they will be very susceptible to H2S toxic gas.
Signal: Weak shrimp swim sluggishly on the water surface or die in the culture period of 25-45 days.
Solution: Always maintain dissolved oxygen levels above 4 ppm. When shrimp are young, all blowers should be kept running regularly. Algae plank floating on the water surface must be removed from the pond and inoculated with PondDtox® microorganisms 2 kg/10,000 m2 into the pond to eliminate toxic H2S gas.
2. Died after 2 months of rearing (70 days to harvest)

Developments: When shrimp were cultured for 70 days, the waste pile in the pond produced a large amount of toxic gas H2S. If there is a problem of sudden lack of oxygen due to algae decay, weather changes (rain, clouds) or overfeeding, anaerobic bacteria will increase their activity and produce a lot of toxic H2S gas. From 70 days onwards, shrimps are susceptible to stress (shock) by toxic gas H2S, other toxic gases, low oxygen content and limited living space. In particular, the weak and those in the molting stage tend to avoid the strong ones into the waste area where there is high H2S toxic gas, causing the shrimp to be stressed (shocked) and become weaker.
Signal: Appears long-lasting bubbles floating on the water in the middle of the pond. The waste turns very black. Sometimes shrimp eat less strongly in the morning. Dark dead shrimp appear with a black mouth and unusually colored gills such as pink, burgundy or black.
Solution: Farmers should plan to stock the right amount of shrimp for each pond. If the above problem occurs, should reduce the amount of food, increase the fan, maintain the pH above 7.5 in the morning, inoculate PondDtox® 2 kg/ 10,000 m2 (1st time) and 1 kg/ 10,000 m2 ( 2nd time) 3-4 days apart.
3. Rain
Developments
Sound: Rain falling on the pond water agitates the shrimp, causing most shrimp to tend to concentrate on the bottom of the pond, where the shrimp will be exposed to waste piles and low dissolved oxygen levels.
Low temperature: When it rains, the pond water temperature drops, shrimp move to a warmer temperature than the waste area and are affected by toxic H2S gas. When the sun returns, the water temperature increases, causing the decomposition of waste to take place at a fast rate. This consumes a large amount of dissolved oxygen in the pond.
Cloudy: It’s cloudy, algae don’t have light to photosynthesize. Algae do not produce oxygen but also use oxygen due to natural respiration. When dissolved oxygen is low, toxic gas H2S is produced and dispersed to the feeding area, causing healthy shrimp to become weak and infected.
Low pH: Rainwater is acidic and that is the cause of the low pH of pond water. Low pH makes H2S more dangerous, H2S gas is extremely toxic (100%) when pH = 5 and non-toxic (0%) when pH = 10.
Pond water stratification: Rainwater causes stratification, fresh water layer above the pond surface and salt water layer below. This interferes with the dissolution of oxygen downward, causing stress (shock) for shrimp and increasing H2S toxicity.
Waves on the water: If it rains with strong winds of 15 km/h or more, it will create strong waves on the surface of the pond water, as well as create a stream of water at the bottom of the pond. This makes the waste pile at the bottom of the pond agitated, peeling off the thin layer of mud that protects the bottom surface, toxic gas H¬2S escapes and covers the entire pond bottom area.
After the rain: When it stops raining, a large amount of waste creates a lot of organic waste and is a food for harmful bacteria to grow. Bacteria consume a lot of oxygen and at the bottom of the waste pile, due to lack of oxygen, desulfurization bacteria produce toxic gas H2S. In addition, the rain made shrimp eat less and more shrimp gathered in the hooves to catch prey rather than at the bottom of the pond. Farmers think that shrimp eat well because they see all the food in their hooves, but in fact, there is still a lot of food left in the bottom of the pond.
Signal: Shrimp become weak, get sick or die after the rain. Some shrimp have dark brown swimming legs, black mouth and darker body color. Shrimp eat less and many die.
Solution: When it rains, it is necessary to maintain the operation of all fans and reduce the amount of feed (about 50%) or wait for 20-30 minutes after the rain stops to feed the shrimp. Timely spread slaked lime (CaO or Ca(OH)2) around the edge of the waste pile. Use PondDtox® microorganisms 2 kg/10,000 m2 before or immediately after it rains to reduce H2S outbreaks. Farmers should regularly check the pH of the water. If the pH is low, apply slaked lime (CaO or Ca(OH)2) around the edge of the waste pile, a little at a time and continue to check and adjust the pH of the water until the pH is above 7.5. Always ensure that the dissolved oxygen concentration at the waste edge is above 4 ppm.
4. Cold weather
Developments: In the waste zone, there are always active bacteria that break down organic matter. This process generates heat, so the waste area is always warmer than elsewhere in the pond. When the weather turns cold, healthy shrimp are more resilient and stay away from waste areas. Meanwhile, weak shrimp with poor cold tolerance will move to the waste area (warmer temperature) and begin to be poisoned by H2S gas. When the weather warms again, the bacteria break down waste at a faster rate and consume large amounts of oxygen. Low oxygen content will make shrimp living in this area poisoned by H2S gas.
Signal: Low temperature slows down the metabolism of shrimp. Shrimp eat less and digest food slowly. Shrimp use less energy and the sensitivity of the sense of smell is reduced. Some shrimp eat less due to toxic gas H2S. When the weather returned to warm, most shrimp ate more, but many died soon after.
Solution: When the weather turns cold (25-26oC), the amount of feed should be reduced by 20-30%. Increase the fan to provide adequate oxygen throughout the day. Place several hooves to check for weak shrimp at the edge of the waste pile. If weak shrimp are detected, then proceed to mix vitamins and minerals into the food to support healthier shrimp. Lime is spread around the edge of the waste pile to keep the pH high and limit the effect of toxic H2S gas.
5. Algae

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Developments: When the algae die, the dead algae is a food source for bacteria and consumes a lot of oxygen. This contributes to increased formation of H2S toxic gas and causes stress (shock) for shrimp, making shrimp more toxic.
Signal: Like in the case of rain.
Solution: When algae die off, it is important to maintain pH through liming (Ca(OH)2). Reduce 30-50% of food in a row for 2-3 days. The water fans need to be maintained throughout the day, because at this time the water fan is the only source of oxygen and the algae are the cause of a lot of oxygen consumption. Farmers should add PondPlus® microorganisms 1 kg / 10,000 m2 into the pond to contribute to the decomposition of algae (PondPlus® is a very good waste decomposer and uses less oxygen).
6. The pond bottom is disturbed
(Due to wading in ponds to prune shrimp, install and repair water blowers, or wade through ponds to pick up dead shrimp from the bottom)

Developments: Normally, the surface of the waste pile is exposed to enough oxygen to create a clean sludge layer. This layer is very thin, has a relatively light color, covers the oxygen-deficient black mud underneath and limits the release of toxic H2S gas in the black mud. But when this surface layer is removed, a large amount of toxic gas H2S suddenly escapes into the water. Nearby shrimps will die from being infected by this toxic gas.

Signal: Shrimp will eat very little in the morning meal or other meals. Dead shrimp will be found after 2-3 days.
Solution:Farmers should not stir up the waste pile at the bottom of the pond, because once a large amount of toxic gas H2S diffuses into the water, it can be difficult to salvage. Shrimp will be affected immediately and treatment at this point becomes expensive. The best that farmers can do is to try to control the pH of the water (liming), cut down on feed (60 – 70%), increase the maximum fan to provide oxygen and immediately introduce PondDtox microorganisms. ® 2 kg/ 10,000 m2 into the pond.
7. Drain the bottom, siphon or change the pond water

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Developments: When draining the bottom or siphon, the thin film covering the waste pile (to prevent toxic H2S from escaping) will be broken and disturbed. Immediately the farmer can smell the rotten egg. A large amount of toxic gas H2S will disperse into the pond water. If the water pH is low, H2S gas will be strongly toxic to farmed shrimp.
Signal: Shrimp stopped eating for 1 or 2 meals. Found some dead shrimp.
Solution: When it is really necessary to clean the bottom of the pond when the shrimp are large, farmers should pay attention not to agitate the surface of the waste pile too hard. If all aerators are installed in the correct position at the start of the crop, the runoff will collect smaller waste piles, and less of a problem will occur. In addition, if using microorganisms, it will help the waste decomposition process take place quickly and maintain a high level of dissolved oxygen in the water, leading to less formation of toxic gas H2S. In case of draining the bottom or siphon of waste, be careful. Before withdrawing the bottom or siphon, farmers should apply lime (Ca(OH)2 or CaCO3) to raise the pH above 8. This makes H2S toxic gas less dangerous to shrimp. At the same time, using PondDtox® 2 kg / 10,000 m2 before withdrawing the bottom or siphon for 1-2 days to help clean up toxic H2S gas.
8. Shrimp molting
Developments: When molting, shrimp are weak and need a lot of oxygen. This is a difficult period, shrimp have to stay away from stronger shrimp so they are forced to hide in the waste area, but unfortunately this area is very oxygen poor and they can be affected by toxic H2S gas.
Signal: After molting, shrimps have soft shell (may also be due to mineral imbalance), weak, easily infected and die. Shrimp are dark or discolored (pale) and have no food in their intestines.
Solution: Farmers should try to manage pond water quality well at all times. Increase the fan to provide enough oxygen when the shrimp is in the molting stage. The fan is also installed in the right position to create a flow that collects all the waste in the middle of the pond.
9. Sudden drop in oxygen

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Developments: H2S gas is extremely toxic because it combines very strongly with blood cells, causing shrimp to not get enough oxygen. Whether in the water lacks oxygen for any reason, such as sudden algae drop, broken fan, cloudy sky for a long time, heavy rain, pond water with salinity stratification will increase the risk. Shrimp muscle is affected by toxic gas H2S.
Signal: Shrimp are weak and infected with unknown disease. Shrimp reduce or stop eating and in the worst case can die.
Solution: Increase the fan to provide adequate oxygen at all times, especially when there is a lack of sunlight, the algae fades, it rains heavily, or the shrimp are molting; If there is a source of clean water, it can be replaced. Check the oxygen content in the pond more often. If pond water changes color, pH should be adjusted at 7.8 – 8.3. H2S toxic gas will become dangerous if the pH drops near 5. According to Bayer Vietnam, 10/10/2015

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Bài viết “Khí độc H2S – “sát thủ giấu mặt” của tôm nuôi” được đăng bởi vào ngày 2022-06-04 03:08:18. Cảm ơn bạn đã đọc bài tại ABC Land – Kênh thông tin BDS uy tín nhất Việt Nam

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