What are the comprehensive prevention and control measures for hemorrhagic disease in farmed fish?

    I. Affected species

    Grass carp, crucian carp, mud carp, common carp, and even silver carp.

    II. Symptoms

    Spotty or patchy hemorrhages on the head, around the eyes, at the base of the fins, and on the abdomen; gill filaments hemorrhagic or pale; sometimes ascites; frayed fin tips.

    III. Causes

    1. Fish diseases caused by bacteria are mostly due to opportunistic pathogens, and will not occur before the aquatic environment has deteriorated. Only when various factors in the substrate and water body become unsuitable for fish survival, reducing the fish's resistance, and at the same time bacteria in the water multiply in large numbers, will the disease break out.

    2. Because fish ponds are fed for long periods, the fish excrete large amounts of feces, and the incompletely digested organic matter pollutes the bottom water; the pond bottom becomes a breeding ground for various bacteria to multiply in large numbers. After multiplying, the bacteria decompose organic matter, and the hydrogen sulfide, nitrite, ammoniacal nitrogen, methane, carbon dioxide and other toxic and harmful gases produced accumulate in the water. If fish live in this for long periods, their constitution declines, giving bacteria an opportunity to infect them.

    IV. Disease prevention measures

    1. The author particularly stresses that to improve fish-farming techniques, one must work hard on the word "management" and not neglect any seemingly minor detail. For fish diseases, one must adhere to the principle of "prevention first, and prevention is more important than treatment," achieving careful management and comprehensive prevention and control.

    2. Injection of the "triple vaccine" for grass carp fingerlings is one of the scientifically effective and important preventive measures.

    3. Thoroughly clear the pond. From the end of each year to February of the following year, water temperatures are relatively low and fish growth is limited. This period can be used to dry the pond and remove excess pond mud. It is best to drain the pond water completely and expose the pond bottom to the sun until it turns white and cracks open with large fissures.

    4. A few days before filling the pond with water, use a large amount of quicklime (in lumps), applying 100-150 kg per mu of pond bottom, or 10-15 kg of bleaching powder, diluted in water and splashed over the whole pond.

    5. Before fingerlings are put into the pond, they should be soaked to disinfect the body surface. Commonly used drugs include 3% salt water, 1/10000 potassium permanganate, 2/10000 formalin, and 4/10000 furazolidone; the soaking time depends on the water temperature.

    6. Reasonable dense stocking and polyculture, making full use of the water body and feed, and not giving the water environment an overload of fish.

    7. Use medium- and high-grade feed, and use less raw materials (raw bran, corn, wheat, yellow powder, etc.) and less low-grade feed. In this way, producing the same amount of fish will discharge less feces, easing the pressure on the pond.

    8. It is recommended to seine the pond once every 20-30 days. Seining and stirring the pond on sunny mornings and at noon brings organic-rich bottom mud up near the water surface, so that some organic matter can be oxidized by oxygen in the water, allows full aeration, and can also inhibit the reproduction of anaerobic bacteria.

    9. After each seining, immediately disinfect with quicklime at 15-20 kg per mu per meter of water depth, or with other water disinfectants; the use of quicklime for disinfection is recommended.

    10. In hot seasons, new water should be added frequently, or part of the pond water replaced.

    11. It is recommended to turn on the aerator at noon on sunny days. In addition to being turned on to rescue fish gasping at the surface, running the aerator for 1-2 hours between 12:00 and 3:00 at noon on sunny days is also an effective measure to ease the pressure on the pond and prevent disease.

    12. Pay attention to pond hygiene, and as far as possible do not discharge unfermented human and livestock manure directly into the pond.

    13. Frequently remove weeds; follow the "four looks" and "four fixeds" principles in feeding; leave no residual feed; dead fish should be taken out of the pond and buried deep in a pit.

    The four looks principle: look at the season, look at the weather, look at the water color, look at the fish's feeding situation.

    The four fixeds principle: fixed location, fixed time, fixed quality, fixed quantity.

    14. Do not raise a single species of fish for many years in a row, so as to improve immunity.