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River Crab Mesh Ecoculture Technology
The Chinese mitten crab (*Eriocheir sinensis*) is a species known for its distinctive physical features. It has a round, cuboidal head, with the posterior part wider than the anterior. Its forehead is equipped with four teeth, with a deep notch between them, and the front margin has four sharp teeth that gradually decrease in size. The chelipeds (claws) are larger in males than females, with fine hairs on the base of the knuckles and fingers. The inner corner of the wrist joint has one sharp tooth, while the dorsal edge of the proximal section and the step foot also feature one sharp tooth. The last three pairs of legs are relatively flat, with bristles on the wrist and back sections. The first pair of walking legs and the base of the knuckles are densely covered with bristles. Female crabs have a rounded abdomen.
**1. Net-based Aquaculture Technology**
This method simulates natural ecosystems by utilizing aquatic plants, snails, and small fish as natural food sources, combined with simple plant-based feeds like corn and wheat. This approach mimics the ecological food chain, creating a wild-like environment that enhances crab quality, reduces disease occurrence, lowers production costs, and improves economic returns. Suitable water areas include reservoirs and lakes with open surfaces, gentle currents, and water quality meeting GB11607-1989 standards. The net enclosure should be placed away from shipping lanes, in quiet and accessible locations. The surrounding area must have abundant aquatic vegetation, high benthic animal density, and low predator pressure. The bottom should be flat, composed of clay or sand, with a consistent water depth of 1–2 meters and a transparency of over 1 meter, along with dissolved oxygen levels above 5 mg/L.
**2. Net Enclosure Setup**
**2.1 Materials**
The perimeter should use polyethylene mesh with a 33-count weave. A top anti-escape barrier is recommended, and gabions can be used at the base to prevent crabs from burrowing. These gabions are cylindrical bags made of 2 cm polyethylene mesh, with a diameter of 10–15 cm and a height of 2–4 cm. Bamboo poles (8–10 cm in diameter, 5 m long) serve as support structures, while polyethylene ropes and iron wire are used for binding.
**2.2 Design**
**2.2.1 Enclosure Area Size**
The net enclosure should typically cover 2–4 hectares, depending on environmental conditions and investment capacity.
**2.2.2 Shape**
The shape can vary—round, oval, rectangular, or triangular—but circular enclosures are the most cost-effective. Square shapes are better for contiguous farming.
**2.2.3 Anti-Escape Measures**
A double-layer net system is used: an internal net with 2 cm mesh to prevent escape, and an external net with 3 cm mesh to block floating debris. The spacing between the two nets is 3–4 meters. The fence height should extend 1.5 meters above the water surface, with a 1-meter-wide raft net attached at the top at a 50–70-degree angle. A 25-cm-high calcium plastic plate is added inside the inner net for extra security, or suspended in the middle of the mesh. The lower edge of the net is sewn with a gabion to prevent burrowing.
**2.3 Installation**
Bamboo piles are driven into the ground every 2–3 meters, with a depth of about 1.5 meters. The net is secured at the top, middle, and bottom of the pile. Gabions are fully embedded in the mud at the base.
**3. Crab Stocking**
**3.1 Source Selection**
Before stocking, predators such as large fish and snakes should be removed. The best source is naturally bred or artificially raised first- or second-generation Yangtze River strains. Crabs should be healthy, uniform in size, active, with intact appendages and no claw damage. Males have incomplete villi on their claws, while females have underdeveloped abdomens. They should weigh between 5–12 grams each, with a density of 80–200 per kilogram.
**3.2 Disinfection**
Crabs are soaked in water for 2–3 minutes, rinsed, and then dipped in 3–5% saline or 10–20 mg/L potassium permanganate solution for 30 minutes.
**3.3 Stocking Methods**
Stocking is usually done from December to February. Density depends on farming methods and management. Typically, 9,000–12,000 crabs per hectare are stocked. They are distributed evenly in areas with plenty of aquatic plants to allow natural movement and reduce competition.
**4. Feeding and Management**
**4.1 Aquatic Plant Transplantation**
If aquatic plants are insufficient, they should be transplanted. Species like *Vallisneria*, *Hydrilla*, *Alternanthera*, and *Abutilon* provide both food and shelter.
**4.2 Feed Types**
Mainly plant-based (aquatic plants, corn, wheat, pumpkin, sweet potato), with supplemental animal feed (fish, snails, prawns).
**4.2.1 Annual Feeding Plan**
Feeding follows a "two thick, one thin" pattern. Start when water temperature exceeds 14°C. Early stages focus on plant feed, increasing to equal parts plant and animal feed as crabs grow. Later stages emphasize animal feed to support weight gain.
**4.2.2 Daily Feeding**
Feed once daily between 4–5 PM. Plant feed is 2–5% of total crab weight, and animal feed is 2–3%. Adjust based on weather, water level, and growth.
**4.2.3 Feeding Methods**
Feed should be evenly distributed to avoid fighting. Corn and wheat should be soaked, while pumpkins and sweet potatoes need to be cooked. Animal feed should be chopped and crushed.
**4.2.4 Regular Sampling**
Monitor feeding and growth through regular sampling to adjust feed quantity and quality.
**4.3 Daily Management**
Newly stocked crabs may climb out initially. Extra cages should be placed between the inner and outer nets for the first few nights. Ensure sufficient aquatic plants (coverage ≥ 1/3), keep the environment quiet, and prevent human or animal disturbance. Water quality should be maintained by removing waste and cleaning the net. Check for escape points regularly, especially during storms or floods.
**4.4 Disease Prevention**
Although this method avoids drugs, prevention is key. Disinfect crabs before stocking, ensure fresh and clean feed, and control predators like large fish, rats, and snakes.
**5. Harvesting**
Harvest occurs between November and December, based on water temperature and market demand. Use traps or other fishing gear. Caught crabs are temporarily held in cages (1.5 × 1.5 × 0.8 m) with a maximum of 50 kg per cage to ensure good condition before sale.