The strategic introduction of crab species into river channels can maximize the potential for water production and enhance economic returns. However, in recent years, some farmers experienced low productivity due to the monoculture of fish. Through continuous exploration and practical experience, a more effective approach—fish and crab polyculture—has proven successful. The average crab yield now reaches 10–15 kg per unit area, and economic benefits have nearly doubled. Below are the key issues and successful practices encountered during the process.
First, selecting an appropriate river ditch is crucial. The site should have clear, fresh, and well-oxygenated water with no pollution, slow-moving currents, and not located on a major waterway. It should be rich in benthic organisms, with dense vegetation covering about 50% of the area.
Second, seed selection and stocking play a vital role. River ditches typically rely on natural food sources, so feeding is minimal or nonexistent. Stocking should follow conventional methods, focusing on the principle of "stocking that year." Grass carp should be stocked based on the amount of aquatic plants to avoid interfering with crab growth. The quality and size of the crabs directly affect survival rates and marketability. In practice, three sizes are commonly used: small, medium, and large crabs. Based on production cycles and efficiency, medium-sized crabs (about 20 grams) are preferred, followed by third-stage crabs. Mature crabs, known as "green crabs," have a dark green shell, widened abdomen, and black hair along the edges, and should be avoided during stocking to prevent early maturity and reduced growth.
Third, proper management is essential. The gully’s water flow and water quality create an ideal environment for crabs, reducing the need for anti-escape structures. However, nets should be placed at both ends of the ditch to prevent migration and escape. Regular checks and fishing activities help monitor crab growth and minimize losses. Bait is rarely used, and daily tasks focus on preventing theft and flood-related escapes.
Fourth, timely harvesting is critical. Crabs should be caught when they reach marketable size, usually around 6–18 months after stocking. Harvesting too early leads to lower profits, while delaying it increases the risk of escape and lower returns. Around mid-September, when temperatures drop and winds shift, increased patrols are necessary to catch any wandering crabs. Various methods, including traps, nets, and screens, can be used effectively.
Finally, intermediate holding is an important strategy. Since most crabs are harvested between September and October, they often lack plumpness, leading to lower prices. To address this, crabs can be held for 2–3 months in designated pools, allowing them to grow and improve market value. For every 100 mu of river ditch, 2–3 acres of holding pools should be prepared. Proper water management, feeding, and monitoring ensure higher income through better timing of market entry.
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