Analysis and Countermeasures of hypoxia in cultured Fish

Hypoxia refers to the state in which the internal tissues and cells of fish do not get enough oxygen to maintain their normal function. In order to distinguish from hypoxia in water, it can be called "biological hypoxia" or "tissue hypoxia". Asphyxia (dyspnea) is a phenomenon that occurs during biological hypoxia, and floating head is a common manifestation of dyspnea or asphyxia in aquaculture production. There are two situations of anoxic asphyxia in fish: one is that when the dissolved oxygen in the water is insufficient and the air pressure is too low, even if the respiratory function of the fish is normal, the tissues and cells in the body cannot obtain sufficient oxygen, resulting in "anaerobic hypoxia"; another situation is that when the respiratory system and circulatory system of the fish are dysfunctional, although the content of dissolved oxygen in the water is not lower than the normal needs of the fish, biological hypoxia and asphyxia will occur.

The main sources of oxygen in pond water are photosynthesis of phytoplankton, dissolution of oxygen in air and supply of water sources, of which photosynthesis of phytoplankton accounts for more than 70%. Oxygen consumption is composed of the respiration of fish and shrimp, the respiration of microorganisms in the water, the oxygen consumption of ponds, and the escape of oxygen from the water, of which the oxygen consumption of microorganisms accounts for about 70%. Phytoplankton are the largest producer of oxygen during the day and the second largest consumer of oxygen after bacteria at night. Therefore, the changes of phytoplankton and sediment in the pond are the main reasons for the change of dissolved oxygen in the pond.

1. Floating head of fish caused by hypoxia in water body

Lack of oxygen in fish can cause severe anemia, slow growth, lighter back color, lip hypertrophy and so on. Asphyxiation occurs when dissolved oxygen falls below its minimum. The requirements of dissolved oxygen in pond water are different in different culture species, different ages and different seasons. Herring, grass carp, silver carp and bighead carp usually float their heads when the dissolved oxygen is less than 1 mg / L and suffocate to death when the dissolved oxygen is less than 0.4 mg / L. the suffocation range of common carp and crucian carp is 0.1-0.4 mg / L. Hypoxia floating head mostly occurs in summer, usually wild mixed fish float head first, common carp and crucian carp begin to float head indicates that hypoxia has been quite serious. The main causes and treatment measures of anoxic floating head are as follows:

(1) the demand for dissolved oxygen in water exceeds the supply. In summer, in case of continuous rain, heavy fog and poor light conditions, the photosynthesis of phytoplankton is weakened and the supply of dissolved oxygen in water is reduced, while the respiration of various organisms and the decomposition of organic matter in the pond consume a large amount of oxygen, resulting in a lack of dissolved oxygen.

Treatment measures: timely injection of new water or start the aerator; rational fertilization, scientific feeding.

(2) anoxic floating head caused by the large temperature difference between the upper and lower water layers. On sunny days in summer, especially some fine ponds, the water quality is thick and fat, and there is a great oxygen difference between the upper and lower water layers during the day. In the afternoon, the oxygen in the upper water is supersaturated (oxygen surplus is produced), and the lower water is seriously anoxic (oxygen debt is generated). Because the water temperature of the upper layer is high and its density is low, it is not easy for the upper and lower water layers to convect. after evening, if the surface water temperature drops sharply in case of thundershowers or strong winds, the density flow occurs, which causes the upper and lower water layers to convection sharply, and the upper water with high dissolved oxygen content rapidly convectively sinks. it is quickly exhausted by the organic matter in the lower water and repay the oxygen debt, while the oxygen content in the upper water can not be replenished, so that the dissolved oxygen in the whole pond decreases rapidly. It is very easy to cause floating head or even flooding pool.

Treatment measures: increase the number of water changes to reduce the fertility of water quality; start an aerator at noon on a sunny day to make the water convection between the upper and lower water layers and increase the amount of dissolved oxygen in the lower water.

(3) the oxygen debt in the fish pond is too large. Fish ponds with thicker silt, uncleared ponds in winter and fish ponds with flooded grass have larger oxygen debt. This kind of fish pond needs to consume a lot of oxygen because there are more reducing substances, so the dissolved oxygen in the pond is insufficient. When the temperature is high, the reducing gas in the sediment sometimes forms bubbles and escapes. This kind of pond can easily cause floating head.

Treatment measures: when the weather is clear, sprinkle quicklime water on the pond to neutralize the organic acid in the sediment; turn on the aerator, on the one hand, increase oxygen, on the other hand, stir the pond water up and down so as to oxidize the bottom reducing substances; pull the empty net to make the pond water mix and flow, promote convection in the upper and lower water layers to eliminate oxygen debt.

(4) the surface of the water is covered with duckweed. A large number of duckweed is distributed on the water surface, which seriously hinders the light from entering the water and weakens the photosynthesis of phytoplankton in the water; at the same time, a large number of duckweed also hinders the dissolution of oxygen in the air and causes floating head. On the other hand, too much duckweed can easily cause fish to lack oxygen and it is not easy to observe the floating head. Some people think that duckweed is a green plant that produces oxygen by photosynthesis, but most of the oxygen produced by duckweed goes to the air.

(5) A large number of phytoplankton died. For example, the application of drugs killed not only zooplankton but also phytoplankton, affecting the production of oxygen. Due to the application of pesticides, the reduction of oxygen in the water results in a decrease in fish food intake on the same day, and hypoxia floating heads may occur the next morning. For example, rotifers or water fleas multiply in large numbers in spring, they filter a large number of phytoplankton to make the water clear, and the respiration of zooplankton is enhanced, and dissolved oxygen is far from meeting the requirements of oxygen consumption.

Treatment measures: eliminate some zooplankton in time, plan the time of application, control the dosage, and strengthen pond patrol after application.

Second, floating head caused by fish disease

(1) Gill rot. Gill rot disease can cause pathological changes in the gills of the respiratory organs of fish, affect its normal respiration and swim in the upper layer of the water body. This kind of float occurs all the year round, whether it is sunny or rainy days, fish float on the surface for a long time, adding new water or turning on an aerator does not sink. If the condition is serious and the weather changes again, it will often cause a large number of deaths.

Treatment measures: for symptomatic medicine, if it is mainly caused by parasites, adopt corresponding insecticides combined with internal administration of antimicrobials according to different types of parasites; if it is bacterial, take internal administration of antimicrobials, at the same time sprinkle fungicides in the whole pool, combined with measures to prevent hypoxia floating head.

(2) A large number of cups are parasitized. It is characterized by a sudden increase in the transparency of the pool water (within 24 hours), common diseased fish swimming slowly on the edge of the pool, seeming to have a layer of hair, and cup worms can be found by microscopic examination. The cups feed on microorganisms in the water. When the cups thrive and devour phytoplankton, the oxygen in the water mainly comes from the dissolution of the air, which is far from meeting the oxygen needs of fish and shrimp. Moreover, because the goblet parasites on the fish gills, it hinders the fish's breathing, resulting in anoxia floating head and even dead fish.

Treatment measures: reasonable fertilization and quicklime should be used to clear the pond to prevent caliciasis. When anoxic floating head occurs, on the one hand, fresh water should be injected, on the other hand, 0.5 × 10 ~ (- 6) copper sulfate and 0.2 × 10 ~ (- 6) ferric sulfate can be sprinkled all over the pool to kill mugworms. After 2 hours, the whole pool was sprinkled with 1 × 10-6 bleach to prevent the injured fish from infection.

Third, the floating head caused by the change of pond water quality

(a) the fish do not adapt to the changes in the water quality of the pond and lack oxygen to float their heads. The main reason is fish poisoning caused by ammonia, nitrite and hydrogen sulfide in pond water. There is no improvement in the implementation of oxygen enhancement measures.

Treatment measures: regularly remove excessive silt and humus from the bottom of the pond; feed reasonably and remove residual bait in time; sprinkle lime water or use photosynthetic bacteria to improve water quality; prohibit polluted water from entering fish ponds.

(2) hypoxia is caused by the change of pH in water environment. The change of pH in water environment can cause corresponding pH changes in the blood of aquatic animals through the penetration and absorption of hydrogen ions, thus destroying its oxygen transport function. Excessive alkalinity will directly corrode the Gill tissue and cause respiratory disorders and suffocation. The change of pH especially has an extremely sensitive effect on the larvae of aquatic animals.

Treatment measures: adjust the water quality to make pH between 6.58.0, the total alkalinity of aquaculture water should be more than 1mm / L.