Common Diseases and Predator-Pest Control Techniques in Freshwater Crayfish Farming

Freshwater crayfish rarely suffer from diseases under natural conditions, but with the expansion of high-density artificial farming, freshwater crayfish farming also faces the risk of disease outbreaks. Therefore, the principle of “prevention first” must be adhered to, and disease prevention and control work must be done well.

(I) Disease prevention 1. Thorough pond clearing and disinfection Before stocking, the pond must be thoroughly cleared and disinfected to kill pathogens and harmful organisms. The inlet water must be filtered to prevent harmful organisms from entering the pond. Introduced shrimp larvae should be soaked and disinfected in 1-3% salt water before being put into the pond. Feed and nets should also be disinfected according to routine procedures. 2. Keep the water quality fresh and clean Regularly flush in new water, or install a matching aerator, to keep the dissolved oxygen content of the pond water above 3 mg per liter; each month, depending on the situation, splash quicklime (20 g per ton of water) over the entire pond to keep the pH value at 7-8. 3. Scientific feeding It is best to use artificial compound pellet feed, and follow the “four fixed” feeding method. Residual feed in the pond or deterioration of water quality and pond bottom are mostly caused by overfeeding. When dissolved oxygen in the water is low, water quality deteriorates, or weather is severe (such as thunderstorm muggy days or consecutive overcast days), feeding should be reduced or stopped. (II) Prevention and control of common diseases and predators 1. Bacterial diseases (1) Brown spot disease. The shrimp's carapace is wounded and infected by various bacteria, which destroy the cuticle layer, epidermal layer, and chitin layer of the shell, forming dark brown spots on the carapace. This disease can generally be shed and self-heal during the next molt, but in severe cases it can also cause shrimp death. (2) Septicemia. It occurs when the shrimp's blood and internal organs are infected by bacteria, including Vibrio, Monosporium, and Gram-negative bacteria. Prevention and control of bacterial diseases: mainly improve water quality and the pond environment, avoid severe irritation and trauma, and focus on prevention. When conditions permit, disinfectants and bactericides can also be applied in shrimp ponds to inhibit bacteria. 2. Fungal diseases (1) Saprolegniasis. Because fungi invade dead crayfish eggs, multiply, and proliferate, the eggs develop white wool-like “saprolegniasis”. In severe cases, the fungi can also infect and invade surrounding healthy eggs, forming patches of white mold. Conventional methods for preventing and controlling saprolegniasis can be tried. (2) Crayfish plague. This disease is caused by infection with a fungus that grows best at temperatures not higher than 18°C; when the water temperature is higher than 21°C, the growth of the fungus is inhibited. Therefore, as long as the water temperature during stocking and harvesting of shrimp larvae is kept above 21°C, crayfish will not contract this disease. 3. Natural enemies of crayfish Birds such as cormorants and gulls are major enemies of shrimp ponds and can be scared away with sound or visual disturbances; fish such as perch, snakehead, catfish, and mandarin fish can prey on shrimp, and crayfish suffer even more when molting, so they should be strictly removed from the pond. If polyculture is desired, fish can only be raised in net cages (sealed net cages) in the pond water. Amphibious animals such as water voles, turtles, snakes, and frogs can also devour crayfish or their young, and can be controlled by trapping and killing or by adding barriers around the pond for isolation. Many aquatic insects or their larvae can also harm young crayfish and can be controlled or trapped, killed, or suffocated by various methods, but special care must be taken with pesticides and other agricultural chemicals used in shrimp ponds, because shrimp are also arthropods and are very sensitive to some pesticides. When bird damage is severe, nets can be used to cover the pond surface to prevent harmful birds from entering the pond to eat shrimp.