In order to optimize the rice field environment for crab farming, careful selection of paddy fields is essential. The fields should be easy to manage for feeding and fortification, with well-constructed ridges that are raised and widened. Typically, the ridge height should be between 50 to 60 centimeters, with a top width of 50 to 60 centimeters as well. A "well" or "tian" type ditch should be dug in the center of the field, with a standard depth of 30 centimeters and a width of 40 to 60 centimeters. A small sink, approximately 6 to 8 square meters in size, with a low entrance and a depth of around 60 centimeters, should be constructed to accommodate crabs during their juvenile stage in autumn. Drainage ditches should be placed in opposite corners of the paddy field, ideally using pipes. The inner end of the pipe should be fitted with a double-layer net to prevent young crabs from escaping and to keep out predators like frogs and voles. Anti-escape walls must be installed before introducing crabs. Two common methods include embedding a fiberglass fence on the inner side of the ridge, standing 60 cm high with 40 cm remaining above ground, or using bamboo stakes and plastic film to create a sloped barrier that tilts inward at 30 degrees, forming semi-circular corners. Additionally, some labor can be introduced into the field to help reduce larvae and improve survival rates.
Feeding management plays a crucial role in the growth of crabs. Early-stage feed includes fresh water grass, fish gills, shrimp stomachs, and cooked pig blood, along with tofu. As crabs grow, additional food sources such as snail meat, stewed meat, soybean meal, crushed rapeseed cake, soybean cake, boiled corn, wheat, pumpkin, and restaurant or slaughterhouse waste can be introduced. During the early stages, feed should be filtered through gauze and provided 3 to 4 times daily, with an amount equal to 30% to 50% of the total weight of the crabs. Later, feeding can be reduced to once a day, preferably in the evening, as crabs tend to feed at night. The feeding amount is typically around 5% to 8% of the total weight of the seedlings, but this may vary depending on factors such as water quality, weather, temperature, and molting cycles. It's important to ensure that crabs are well-fed without overfeeding.
Water quality management is key to maintaining healthy crabs. Freshwater should be added when crabs are first introduced, with an initial depth of 10 cm. As water temperature rises and crabs grow, the water level should gradually increase to 15 to 20 cm. Water should be changed weekly, with each change involving about 5 to 10 cm of water replacement to maintain freshness and high dissolved oxygen levels. Lime should be applied monthly at a rate of 20 kg per acre to keep the water slightly alkaline, which helps in sterilization, disinfection, and calcium supplementation. Based on water conditions, organic manure (100–130 kg/acre) or urea (5–7 kg/acre) can also be used. If possible, maintaining a slight flow in the paddy field can stimulate appetite and increase shell production.
Disease prevention and control are vital in ensuring the health of crabs. Young crabs are vulnerable to predators such as waterflies, snakes, and especially mice, which can kill up to 20–30 crabs overnight. Effective measures include rodent control and manual removal. Aquatic plants should be soaked in lime water (20–30 g/m³) and periodically treated with quicklime to disinfect the water and prevent disease. When applying pesticides, it is essential to use high-efficiency, low-toxicity, and low-residue options, applied in portions to minimize contamination. After pesticide application, water should be replaced to ensure it remains safe. Juveniles are typically harvested before rice cutting, usually in mid to late September.
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