Rice-field crabs are an excellent method for integrating rice, fish, and crabs in a mutually beneficial and symbiotic system. To ensure a successful harvest, several key practices must be carefully followed.
First, selecting the right field is crucial. The area should have good water quality, sufficient water supply, easy access, and good water retention. It should also be a low-lying, contiguous area with shallow zones. The breeding project should include trenches, ditches, storage ponds, and raised ridges around the field. These structures should be interconnected, with the crab farming trenches typically 3 to 6 meters wide and 1 meter deep. The spacing between trenches should be about 15 to 30 meters. Before use, the trenches must be disinfected, and two layers of anti-escape nets should be installed at the inlet and outlet. An escape prevention wall should be built around the area, with rounded corners. The wall should extend at least 20 cm above the ground and have a smooth inner surface to prevent crabs from climbing out.
Second, choosing the right plant species is important. Submerged plants like Vallisneria and Potamogeton edulis should be planted in the ponds, while floating or emergent plants such as water lilies and lotus can be added. For crabs, "yellow crabs" with uniform size and strong vitality are ideal. Stocking density depends on the planned yield, pond size, feed availability, and management level. To grow larger crabs within a year, stocking density should be slightly reduced, aiming for a weight gain of 5 to 7 times. Small fish like tilapia or silver carp can also be added to improve water quality and increase overall yield. For example, in a 100 square meter area, stocking 4 to 5 kg of crabs per 120–160/kg and 6–8 kg of fish can result in a harvest of about 25 kg each by year-end, significantly boosting productivity. If crabs are to be stocked, they should first be raised in holding ponds before being transferred to the main field.
Rice varieties should be short-season, disease-resistant, and able to withstand fertilizers. Large seedlings should be transplanted with wide spacing to allow for better growth. Fish and crabs should be introduced into the field only after the rice has established itself.
Third, feeding and management play a vital role. Crabs should be fed according to their growth stage, with a balanced mix of fine, green, and animal-based feeds. Feeding amounts should gradually increase with weight and temperature. Feeding should be done in small portions, at fixed times, and with precise quantities. About 30% of the daily feed should be given in the morning, and 70% in the evening, leaving a small amount for the next day. When the water temperature is below 18°C, feed should be about 5% of the crab’s body weight; above 18°C, it increases to around 11%. In the early stages, crabs rely more on green materials, while in later stages, they require more protein-rich feed for fattening. Controlling premature maturation is essential to promote larger individual growth. Cost-effective feed sources should be developed to reduce expenses.
Regular water quality monitoring is necessary. Water should be changed every 2–3 days, with a quarter of the water replaced each time or 20–30 cm added. Observing crab behavior during patrols helps detect issues early. Maintaining clean water and preventing escapes, pests, and diseases is critical. Rats not only eat crabs but also create burrows that can lead to escape. Optimizing the environment reduces burrowing activity and supports healthy growth.
For rice management, the focus should be on ensuring robust plant growth. Heavy basal fertilization, light top-dressing, and no pesticides are recommended. If pesticides are unavoidable, choose low-toxicity, high-efficiency options and apply them in small amounts, in patches, and on leaves. After pesticide use, increase water exchange to reduce chemical residue. Urea can be used for nitrogen top-dressing.
Based on successful experiences, rice-field crab farming has minimal impact on rice yield and reduces the need for pesticides. Thanks to the crabs' natural feeding habits, chemical fertilizer use is also lowered. Therefore, under the condition that rice yield remains stable compared to traditional rice cultivation, significant cost savings can be achieved in terms of pesticides and fertilizers per mu (approximately 0.067 hectares).
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