Analysis of the Causes of Hypoxia in Farmed Fish and Countermeasures

Hypoxia refers to a state in which the internal tissues and cells of a fish cannot obtain sufficient oxygen to maintain their normal functions. To distinguish it from oxygen deficiency in the water, it may be called "biological hypoxia" or "tissue hypoxia." Asphyxia (difficulty breathing) is a phenomenon that occurs during biological hypoxia, and "floating head" (gasping at the surface) is the manifestation of the difficulty breathing or asphyxia commonly seen in aquaculture production. There are two situations in which fish experience hypoxic asphyxia: one is when dissolved oxygen in the water is insufficient and air pressure is too low, so that even if the fish's respiratory function is normal, the tissue cells in its body cannot obtain sufficient oxygen, resulting in "anoxic hypoxia"; the other is when functional disorders occur in the fish's respiratory and circulatory systems, so that even though the dissolved oxygen content in the water is not below the fish's normal requirement, biological hypoxia and asphyxia still occur.

The oxygen in pond water comes mainly from three sources: photosynthesis by phytoplankton in the water, dissolution of oxygen from the air, and replenishment from water sources. Among these three, photosynthesis by phytoplankton accounts for more than 70% of oxygen production. Oxygen consumption is composed of respiration by fish and shrimp, respiration by microscopic organisms in the water, oxygen consumption by the pond bottom substrate, and escape of oxygen from the water, of which oxygen consumption by microscopic organisms accounts for about 70%. Phytoplankton is the largest oxygen producer during the day, but at night it becomes the second-largest oxygen consumer after bacteria. Therefore, changes in phytoplankton and the condition of the bottom substrate in a pond are the main causes of changes in dissolved oxygen in the pond.

I. Fish floating head caused by oxygen deficiency in the water body

Hypoxia in fish can cause severe anemia, slow growth, fading of body color on the back, and enlarged lips. When dissolved oxygen falls below its minimum limit, it will cause asphyxiation and death. Different cultured species, different ages, and different seasons have different requirements for dissolved oxygen in pond water. Black carp, grass carp, silver carp, and bighead carp usually begin to gasp at the surface when dissolved oxygen in the water falls below 1 mg/L, and die of asphyxiation when it falls below 0.4 mg/L; the asphyxiation range for common carp and crucian carp is 0.1-0.4 mg/L. Hypoxic floating head occurs mostly in summer. Usually wild and miscellaneous fish gasp at the surface first; when common carp and crucian carp begin to gasp at the surface, it indicates that hypoxia is already quite severe. The main causes of hypoxic floating head and the treatment measures are as follows:

(1) The demand for dissolved oxygen in the water exceeds the supply. In summer, during continuous overcast and rainy weather, heavy fog, or poor light conditions, the photosynthesis of phytoplankton weakens, and the amount of dissolved oxygen replenished in the water decreases, while the respiration of various organisms in the pond and the decomposition of organic matter consume large amounts of oxygen, resulting in insufficient dissolved oxygen.

Treatment measures: Promptly add fresh water or start aerators; fertilize reasonably and feed scientifically.

(2) Hypoxic floating head caused by a large temperature difference between the upper and lower water layers. On sunny days in hot summer, especially in some intensive@#@233@#@ ponds, the water quality is thick and fertile. During the day, the oxygen difference between the upper and lower water layers is very large. By the afternoon, the upper layer of water is supersaturated with oxygen (producing oxygen surplus), while the lower layer of water is severely oxygen-deficient (producing oxygen debt). Because the upper layer of water has a higher temperature and lower density, the upper and lower water layers do not readily convect. After evening, if there is a thunderstorm or strong wind, the surface water temperature drops sharply, producing a density current that causes rapid convection between the upper and lower water layers. The upper layer of water with higher dissolved oxygen quickly convects downward and is rapidly depleted by organic matter in the lower water layer, repaying the oxygen debt, while the oxygen content of the upper layer cannot be greatly replenished. As a result, the dissolved oxygen content of the entire pond drops rapidly, very easily causing floating head or even a pond turnover.

Treatment measures: Increase the frequency of water exchange to reduce the fertility of the water quality; start aerators at noon on sunny days to cause convection between the upper and lower water layers and increase the dissolved oxygen content in the lower layer of water.

(3) Excessive oxygen debt in the fish pond. Fish ponds with thick silt, fish ponds that were not cleared in winter, and fish ponds where water was released to flood grass for fish culture all have relatively large oxygen debts. Because such fish ponds contain many reducing substances that need to consume large amounts of oxygen, the dissolved oxygen in the pond is insufficient. When the temperature is relatively high, reducing gases in the substrate sometimes form bubbles and escape. Such ponds are prone to causing floating head.

Treatment measures: On clear days, splash quicklime water over the pond to neutralize organic acids in the substrate; start aerators to both increase oxygen and stir the pond water up and down so that reducing substances at the bottom are oxidized; pull an empty net through the pond to mix and circulate the pond water, promoting convection between the upper and lower water layers and thereby eliminating the oxygen debt.

(4) Dense duckweed covering the water surface. Large amounts of duckweed distributed on the water surface seriously obstruct the entry of light into the water body, greatly weakening the photosynthesis of phytoplankton in the water; at the same time, large amounts of duckweed also obstruct the dissolution of oxygen from the air, causing floating head. Too much duckweed also makes it difficult to observe when fish are floating at the surface due to hypoxia. Some people think that duckweed, being a green plant, will carry out photosynthesis and produce oxygen, but most of the oxygen produced by duckweed goes into the air.

(5) Mass death of phytoplankton. For example, applying chemicals not only kills zooplankton but also kills phytoplankton, affecting oxygen production. Due to chemical application, reduced oxygen in the water causes fish to eat less that day, and the next morning hypoxia and floating head may occur. Another example is when rotifers or water fleas reproduce in large numbers in spring; they filter-feed heavily on phytoplankton, making the water clear, and the respiration of zooplankton increases, so dissolved oxygen is far from meeting the oxygen consumption demand.

Treatment measures: Promptly eliminate part of the zooplankton; plan the timing of chemical application and control the dosage; after application, patrol the pond more intensively.

II. Floating head caused by fish diseases

(1) Gill rot disease. Gill rot disease can cause lesions in the gills, the respiratory organs of fish, affecting their normal breathing and causing them to swim in the upper layer of the water body. This type of floating head occurs in all four seasons. Whether on sunny days or cloudy and rainy days, the fish remain floating at the surface for a long time, and adding fresh water or starting aerators does not make them sink. If the disease is severe and is accompanied by sudden weather changes, it often causes mass mortality.

Treatment measures: Use medication according to the symptoms. If it is mainly caused by parasites, use corresponding insecticides according to the type of parasite and combine with oral antibacterial drugs; if it is bacterial, use oral antibacterial drugs, and at the same time splash bactericidal drugs over the whole pond, combined with measures to prevent hypoxic floating head.

(2) Heavy parasitism by Epistylis. Its characteristic is that the transparency of the pond water suddenly increases (within 24 hours). Diseased fish are commonly seen swimming slowly near the pond edge, with what appears to be a layer of hair-like material on their bodies; at this time, microscopic examination can reveal Epistylis. Epistylis feeds on microorganisms in the water. When Epistylis reproduces vigorously and consumes large amounts of phytoplankton, the oxygen in the water comes mainly from dissolution from the air, which is far from sufficient to meet the oxygen needs of fish and shrimp. Moreover, because Epistylis parasitizes the fish gills, it obstructs the fish's breathing and leads to hypoxic floating head or even fish death.

Treatment measures: Fertilize reasonably, and at the same time use quicklime to clear the pond to prevent Epistylis disease. When hypoxic floating head occurs, on the one hand add fresh water, and on the other hand splash a solution of 0.5×10-6 copper sulfate and 0.2×10-6 ferric sulfate over the whole pond to kill Epistylis. After 2 hours, splash 1×10-6 bleaching powder over the whole pond to prevent infection of damaged fish bodies.

III. Floating head caused by changes in pond water quality

(1) Fish cannot adapt to changes in pond water quality and develop hypoxic floating head. The main cause is poisoning of fish by ammonia, nitrite, and hydrogen sulfide in the pond water. Oxygen-increasing measures also show no improvement.

Treatment measures: Regularly remove excess silt and humus from the pond bottom; feed reasonably and promptly remove residual bait; splash lime water or use photosynthetic bacteria to improve water quality; prohibit polluted water bodies from entering fish ponds, etc.

(2) Hypoxia caused by changes in the pH of the water environment. Changes in the pH of the water environment can, through the permeation and absorption of hydrogen ions, cause corresponding pH changes in the blood of aquatic animals, thereby damaging their oxygen transport function. Excessive alkalinity will directly corrode gill tissue, causing respiratory obstruction and asphyxia. Changes in pH have an especially sensitive effect on the larvae of aquatic animals.

Treatment measures: Adjust the water quality so that the pH is between 6.5-8.0, and the total alkalinity of the aquaculture water body should be greater than 1 mmol/L.