Aquaculture Measures to Improve the Survival Rate of Two-Year-Old Grass Carp Fingerlings
I. Main Reasons for the Low Survival Rate of Two-Year-Old Grass Carp Fingerlings
1. Pond not cleared or not thoroughly cleared before stocking Thoroughly inadequate pond clearing easily leads to poor pond bottom quality, with large amounts of pathogenic bacteria remaining on the pond bottom. When the weather is erratic as it has been this year and water quality is unstable, this will cause sporadic deaths of the fingerlings.
2. Stocking fingerlings after pond clearing without first fertilizing the water According to investigations, many farmers stock fingerlings right after clearing the pond without fertilizing the water first. As a result, water quality changes quickly and becomes unstable, causing new stress in two-year-old grass carp fingerlings. At the same time, after the grass carp fingerlings are put into the pond, there are few suitable natural food organisms, so they grow slowly, have poor constitutions and are prone to illness, resulting in a low survival rate after grow-out and continuous deaths.
3. Poor water environment Because the pond water quality is unstable and the climate varies greatly, grass carp are prone to diseases such as hemorrhagic disease and gill rot, resulting in a low survival rate. This is especially true in ponds fed with pelleted feed: as the fish load in the pond increases and the amount of feed fed rises, water quality becomes even harder to control and is prone to aging and deterioration. The levels of harmful substances such as ammonia nitrogen, nitrite and hydrogen sulfide rise rapidly, causing chronic poisoning of the fingerlings and resulting in continuous deaths of two-year-old grass carp fingerlings.
4. Unreasonable feed feeding Many farmers prefer to feed high-protein feed, believing that high-protein feed will increase the growth rate of grass carp, while neglecting the grass carp's own nutritional needs and nutritional balance. In addition, the vitamin content in the feed is insufficient, especially vitamin C, and the result is often the occurrence of nutritional diseases in the fish (such as fatty liver, liver degeneration, etc.).
5. Unreasonable disease prevention measures Some farmers have poor awareness of disease prevention and rely excessively on and trust in the role of drugs. During the breeding process, they still use traditional disease prevention methods, often adding some oral antibacterial drugs to the feed to prevent disease, while neglecting the fundamental role of improving the water ecological environment and the scientific use of drugs. The result is that, due to excessive medication, pathogens develop strong drug resistance, and the internal organs of grass carp fingerlings (such as liver, kidney, spleen, etc.) are severely damaged, greatly reducing the grass carp's own digestion, detoxification and disease resistance abilities.
II. Measures to Improve the Survival Rate of Two-Year-Old Grass Carp Fingerling Culture The growth of two-year-old grass carp fingerlings has high requirements for the water environment. Especially under intensive pond culture conditions, every link in the culture process must be handled well, and ecological culture and healthy culture should be advocated. Specific measures are as follows:
1. Thorough pond clearing and disinfection The pond is the basic environment on which fish depend for survival, and thorough pond clearing and disinfection is the foundation for creating a good aquaculture ecological environment. When the bottom quality is good, the water quality is naturally good, but good water quality does not necessarily mean good bottom quality. Therefore, pond clearing and disinfection are crucial; it is one of the “most critical” measures in ecological healthy fish farming technology and successful aquaculture. For pond clearing and disinfection, first, excessive silt must be removed, the pond must be thoroughly sun-dried, the bottom quality improved, and leaks sealed to prevent seepage (to prevent virus “serial” transmission), creating a good “living” environment for healthy fish growth; second, disinfectant drugs should be applied to kill harmful organisms in the pond. The specific operation is to disinfect the pond with drugs 10-15 days before stocking. First retain 10-20 cm of water at the pond bottom, and use 150-200 kg of quicklime, applied freshly mixed. After 4-5 days, wash away the quicklime water, then fill with new water. A filter screen should be installed at the water inlet to filter out harmful organisms and impurities. Then 200-300 g/mu of bromochlorohydantoin can be splashed over the whole pond to quickly and thoroughly kill pathogens and harmful organisms.
2. Scientific fertilization management Before stocking fingerlings, use bio-fermented organic fertilizer to fertilize the water and cultivate natural food organisms, so that the water appears yellow-green or tea-brown. The water should be rich but fresh, creating a good algal phase, with transparency maintained at 30-40 cm and pH at 7.5-8.0, so that there is enough live food in it. At the same time, food organisms, especially plankton, play an important role in purifying water quality, absorbing harmful substances such as ammonia nitrogen and hydrogen sulfide in the water, and stabilizing water quality. Abundant algae are a sign of enhanced water self-purification capacity. Only abundant algae can fully utilize the products of microbial degradation. Therefore, algae are the main body of plankton in water bodies, and the lifespan of algae themselves is only 10-15 days. The normal growth of algae (including live bacteria) requires nutrient elements such as nitrogen, phosphorus, iron, zinc, copper, cobalt, magnesium and potassium, as well as various vitamins. Therefore, biological water-fertilizing agents need to be splashed regularly to supplement nutrients. At the same time, note that large water exchanges should not be made, as they will cause algae to lack sufficient nutrient supply and die faster. Poor reproduction of algae and live bacteria will cause unstable water quality. Therefore, water exchange should be reduced and nutrient sources appropriately supplemented, so as to keep algae stable in the pond and maintain the ecological balance in the pond.
3. Necessary disease prevention measures before fingerling stocking Before stocking two-year-old grass carp fingerlings, they must be injected with 0.2-0.5 ml per fish of grass carp hemorrhagic disease inactivated vaccine diluted 10 times, or soaked with a special vaccine for grass carp. The method is to place the fingerlings in a solution of 0.5% inactivated vaccine plus 1.0 mg/L scopolamine for 2-3 hours to prevent viral hemorrhagic disease of grass carp during the culture process. Alternatively, soak the fingerlings in 5-6 mg/L amino acid iodine solution for 15-20 minutes to kill viruses and bacteria that the fingerlings themselves may carry and to enhance the immunity and antiviral ability of the fingerlings.
4. Improve the feed nutrition of two-year-old grass carp fingerlings Feed high-quality feed and feed reasonably to ensure the fingerlings eat evenly, eat enough and eat well. Prepare reasonable special feed according to the nutritional needs of grass carp fingerlings, and add high-quality multivitamins and minerals to prevent vitamin deficiency. At the same time, throughout the entire culture process, according to the living habits and disease occurrence characteristics of grass carp fingerlings, reasonably adjust the feeding amount of compound feed and appropriately increase the feeding amount of various high-quality aquatic grasses. Before the season of frequent fish diseases arrives, try to feed well, but do not overfeed; 80% full is sufficient.
5. Strengthen aeration measures Sufficient oxygen is a necessary condition for stable water quality and fish growth. During fish culture, as culture time increases, the accumulation of bottom sediment in culture ponds makes heterotrophic bacteria at the pond bottom the dominant bacterial group, causing severe oxygen deficiency at the pond bottom, which in turn causes nitrite and ammonia nitrogen to accumulate due to incomplete oxidation (poisoning occurs). The most serious consequence of bottom oxygen deficiency is the massive proliferation of pathogenic bacteria such as Aeromonas hydrophila and Vibrio. Combined with the fact that oxygen deficiency has already significantly reduced fish immunity, disease outbreaks become extremely likely. Therefore, in order to reduce the bottom sediment stock to a minimum, dissolved oxygen must be high enough to drive away and oxidatively decompose the sediment and provide conditions for broad contact for biodegradation of the sediment. The most effective means is to improve water circulation and eliminate bottom oxygen deficiency. The method is to splash an oxygen-increasing agent mainly composed of sodium percarbonate at 200-300 g/mu over the whole pond during muggy weather, rainy days and regular nighttime from 24:00 to 1:00, while continuously running the aerator.
6. Strengthen water quality regulation technology Grass carp fingerlings prefer to live in relatively fresh water quality environments, so various measures should be taken during the culture process to strengthen water quality regulation and management. During the culture process, especially in the middle and late stages of culture, due to increased feeding amount, increased residual feed, fish feces and the remains of small organisms accumulating over time, the bottom quality further deteriorates, making outbreaks of disease very likely. Therefore, use regular, quantitative splashing of microecological preparations such as photosynthetic bacteria, nitrifying bacteria, Bacillus, EM bacteria, etc., and appropriate water exchange to regulate water quality, maintain good water quality from beginning to end, improve dissolved oxygen, stabilize the algal phase, reduce pH fluctuations, lower the content of harmful substances such as ammonia nitrogen, nitrite and hydrogen sulfide, keep the water environment relatively stable, make beneficial bacterial species in the water body the dominant population, inhibit the growth of harmful microorganisms, and thereby reduce the breeding of pathogens. Control the ammonia nitrogen content of the water body below 0.5 mg/L, nitrite content below 0.05 mg/L, and water transparency above 30 cm.
7. Improve disease prevention measures Scientifically use disinfectant drugs to improve the immunity and disease resistance of fish. In recent years, many applications have shown that choosing Chinese herbal medicines or Chinese herbal extracts such as baicalin polysaccharide and ginsenoside immunostimulants has definite effects on improving fish immunity and disease resistance. In view of the continuous deaths occurring during the culture of two-year-old grass carp fingerlings, medicated bait composed of Chinese herbal medicines such as Coptis chinensis, Phellodendron amurense, Astragalus membranaceus, Isatis root, Artemisia capillaris and licorice can be fed, supplemented with drugs such as Vc and VE to protect and support the liver. At the same time, 150-200 g per mu of Sanbao high-stability vitamin C can be splashed to improve the anti-stress ability of fish. Disinfectants should be those with low irritation, such as amino acid iodine, chlorine dioxide, bromochlorohydantoin, etc., to prevent abuse of drugs, damage to water quality and new stress to fish. The above are some basic measures to improve the survival rate of two-year-old grass carp fingerlings. Because two-year-old grass carp fingerlings are often affected by factors such as density, environmental conditions and growth rate during intensive pond culture, in order to improve their survival rate, beneficial biological preparations should be fully utilized to improve pond water quality and the pond environment, and pond aeration measures and stress management should be strengthened, so that two-year-old grass carp fingerlings live in a comfortable environment, which is conducive to their growth and improves the survival rate of two-year-old fingerlings.