What is the relationship between diseases of eels raised in earthen ponds and the water environment?
The water environment is the decisive factor in eel farming. The water environment not only determines whether eels can be farmed, but also directly affects the level of production and ultimately the economic benefits of farming. Therefore, only by improving the water environment can eels be farmed well. The following analysis focuses on the relationship between eel diseases and the water environment from the following aspects.
1. pH level
The pH value in water is an important comprehensive water quality indicator, and it directly affects the physiological condition of eels. When the biological, chemical or physical components in the water body change, they will all be reflected in the pH value. Eels in earthen ponds require the pond water pH to remain between 6.5 and 8.5. If the pH value of the water body is less than 6.5, it is acidic, which will cause acidosis in eels, cause protein denaturation, make tissues and organs lose function and lead to death; when the pH value is greater than 9.0, the water body is alkaline and has strong corrosive effects on eels, causing severe gill damage. Water that is too acidic or too alkaline can stimulate the sensory nerve endings of the eel's gills and skin, reflexively affecting respiratory movement and weakening the eel's ability to take up oxygen from the water body. Therefore, even in oxygen-rich water, eels will still show symptoms of hypoxia.
Control measures: when the pH value is less than 6.5, 30 grams/cubic meter of quicklime can be used for adjustment, or "microecological accelerator" can be used to raise the pH value. If the pH value is greater than 9.0, the water should be changed immediately, or bromochlorohydantoin, dibromohydantoin or monobasic stabilized chlorine dioxide can be splashed over the whole pond at a concentration of 0.2-0.3 grams/cubic meter.
2. Dissolved oxygen
Dissolved oxygen in the water body is an important environmental condition for the survival and growth of eels, and it mainly comes from the photosynthesis of phytoplankton. Low oxygen is very unfavorable to the growth of eels. In mild cases, it affects feeding, growth and feed utilization; in severe cases, it leads to decreased disease resistance and death from disease. The dissolved oxygen requirement for eels should remain above 4 mg/L. Causes of low dissolved oxygen: first, high temperatures, eels have high activity and consume more oxygen; second, excessive stocking density, eels have high respiratory demand; third, decomposition of organic matter in the pond and oxidation consumption of inorganic matter, etc.
Control measures: reasonably match mixed culture species and stocking density. The most ideal stocking density for earthen ponds is: for 20P/kg, 2500-3500 fry per mu; for 30P/kg, 3000-4000 fry per mu. Management personnel are required to measure dissolved oxygen regularly every day. If insufficient dissolved oxygen in the water body is found, aerators should be used in time. The duration of aerator use should be determined according to weather changes. If the water color is indeed aging, fresh water should be added in good time or 80-150 grams of monobasic stabilized chlorine dioxide should be splashed per mu.
3. Water temperature
Water temperature directly affects the metabolic intensity of eels, thereby affecting their feeding and growth. The suitable temperature range for eels in earthen ponds is 20-28°C. Within this range, the metabolism of eels is correspondingly enhanced, feed intake increases, and growth accelerates. Conversely, eel metabolism weakens, immunity declines, and disease or even death occurs. On the other hand, various pathogenic microorganisms reproduce under certain water temperature conditions, thus leading to disease outbreaks. For example, European eel restless disease generally occurs in high summer temperatures, while saprolegniasis is prevalent when water temperatures are low in winter and spring.
Control measures: ① Feed fresh and high-quality bait; ② When the climate changes abruptly and water temperature and dissolved oxygen fluctuate greatly, the amount of bait should be reduced, and attention should be paid to water quality regulation. During the entire farming period, the "three adjustments" measures should be adopted, namely adjusting freshness, adjusting quality and adjusting vitality; ③ During the peak feeding season of eels, some stomach-benefiting powder, liver-protecting and gallbladder-nourishing granules, bacteria-toxin powder, multiple vitamins, allicin or Chinese herbal preparations and other digesting aids and anti-inflammatory and detoxifying preparations should be added and mixed into the feed for oral administration.
4. Thoroughly clean the pond bottom and sludge
Cleaning the pond bottom and sludge is the foundation for creating a good farming ecological environment. Pond organic matter and humus sludge are often the main source of deterioration and induced disease outbreaks. Studies show that excessive sludge easily deteriorates water quality, increases acidity, and makes it easy for pathogens to reproduce in large numbers. At the same time, in a poor environment, the resistance of eels weakens, so diseases are prone to occur.
Control measures: newly built earthen ponds should be filled with water to soak before releasing fry, with the water depth maintained at 50-70 cm, and the water changed 2-3 times within a week, with each addition of water not exceeding 20 cm, so that the pH value stabilizes between 7.0 and 8.4. If it is an old pond, the sludge must be strictly and thoroughly cleaned, plowed or exposed to the sun. If conditions permit, it is best to first use a bulldozer to push out 10-20 cm of bottom mud and transport it outside the dike, and then, in addition to applying 75-100 kg of quicklime per mu, apply 0.1-0.2 kg of bleaching powder per cubic meter of water body, or apply 0.1-0.2 kg of monobasic stabilized chlorine dioxide per mu and splash it over the whole pond, to kill harmful organisms. After disinfection, the earthen pond should be soaked with water filtered through a 60-80 mesh sieve, drained after 3-5 days, repeated 3-5 times, and after thorough disinfection, new water should be filled in (newly built earthen ponds are disinfected in the same way), and 5‰-7‰ salt should be added to the new water to prevent virus transmission.
5. Ammonia nitrogen, hydrogen sulfide and nitrite
Ammonia comes from pond bottom mud and large amounts of organic fertilizer entering the pond as well as the excrement of eels themselves, and is produced through nitrification. It is a highly toxic substance to eels, can destroy gill tissue, and penetrates into the blood, reducing the oxygen-carrying capacity of the blood and lowering respiratory function.
Hydrogen sulfide comes from sulfate-reducing bacteria in the pond bottom sludge reducing sulfate, and from heterotrophic bacteria decomposing organic sulfides in residual bait or feces, causing the sludge to turn black. This is an important sign of the presence of hydrogen sulfide.
Nitrite is a product of ammonia during nitrification. It is an important cause of inducing eel diseases. The reason is that in summer, when temperatures are high, large amounts of dead animal and plant bodies, eel feces and residual bait deposits decompose at the pond bottom, producing toxic substances such as ammonia nitrogen, hydrogen sulfide and nitrite, which deteriorate water quality and cause hypoxia or poisoning death in eels. In addition, due to abuse of drugs in ponds or improper introduction of water sources, organophosphate poisoning is very easily caused. Control measures: for farming ponds with a water depth of 1-1.5 m, splash monobasic stabilized chlorine dioxide or photosynthetic bacteria over the whole pond at 0.1-0.2 kg per mu, generally 1-2 times per month. This drug can not only effectively kill these pathogenic microorganisms, but can also react with hydrogen sulfide, organic matter, etc. in the water body, thereby achieving the purposes of deodorizing, inhibiting bacteria and disinfecting, reducing ammonia and increasing oxygen, improving bottom conditions and purifying water quality, and preventing the occurrence of various diseases.