The use of purse seining in Lake Yangcheng Lake, located in Bacheng, Kunshan, began in 1987. Initially, the area was just around 1,000 acres. By 2000, it had expanded to over 25,000 acres. This article provides an overview of the techniques used for crab farming in the lake and the implementation of purse seine methods in the region.
First, water selection is crucial for aquaculture. It is recommended to choose freshwater lakes with clear water, slow-moving currents, and a depth of about 1.5 meters. The lake bottom should be hard and flat, without deep trenches, and the mud layer should be around 10 cm thick. These lakes should also have abundant natural aquatic resources and a rich supply of benthic organisms.
Next, preparing the fencing materials is essential. The mesh size should be 12 shares with 8–9 holes per inch. Bamboo poles, approximately 20–25 cm in diameter, should be used as posts, ensuring they are straight and undamaged. The bottom gabion is made from 246 stones, while other accessories include Moso bamboo, plastic lines, ropes, and lead wires.
The fence area is typically between 30 to 50 acres, and the shape is usually square or rectangular. When setting up the fence, it's best to do so at the end of the year or in January. Choose a flat, trench-free area, and place the piles 1 meter apart. Reinforce the fence to prevent damage from strong winds.
The height of the fence should be 80 to 100 cm above the water level during flood season to avoid flooding. A counter-netting net should be installed at the end of the fence to prevent crabs from escaping. This net should be 50 cm wide and angled inward at 45 degrees. Ensure that the connection between the net and the bamboo poles is slightly loose to avoid damage from wind.
For the gabion setup, the main gabion should have a diameter of 10–12 cm, filled to about 70–80% to allow easy placement. An auxiliary gabion with a diameter of 5–6 cm should be placed inside, 50 cm away from the main one, to prevent crabs from escaping if the main gabion is damaged.
After all the fences are set up, a cage net should be placed on all four sides to monitor for any crab escapes. Regular inspections help ensure the integrity of the system.
In terms of breeding management, stocking should occur between March and April. Healthy, disease-free crabs weighing 60–80 per kilogram are selected. Stocking density is generally around 400 crabs per acre. Before releasing them, crabs should be sterilized using potassium permanganate at a concentration of 10–15 mg/L for 1–2 minutes until they show a red color when spitting water.
Supplemental feeding begins in early April, with fresh snails added at 300–500 kg per mu. This helps increase natural food sources and clean up debris left by the crabs. Artificial feeding is also used, with fresh fish, cooked corn, and wheat provided after April when water temperatures rise.
From June to August, non-polluting drugs are regularly added to the bait to prevent and treat diseases. Feeding amounts should be controlled so that crabs consume the food within two hours to avoid sediment spoilage and potential health issues.
Daily management involves checking the nets for escapes, inspecting the fence structure, and monitoring the environment. Morning and evening checks are conducted to remove debris, assess plant growth, and adjust feeding based on crab activity.
Harvesting occurs around late September when crabs reach maturity. Methods include using large-knotted nets in the fences, with 2–3 nets per mu. Another method is to catch crabs on the anti-escape nets, achieving an average recovery rate of 70%, with crabs averaging 150 grams each.
In terms of profitability, selling 40 kg of crabs at 210 yuan per kilogram generates 8,400 yuan in revenue. Costs include 1,000 yuan per mu for nets over three years (330 yuan annually), 400 yuan for crab stock, 600 yuan for feed, 200 yuan for rent, 800 yuan for labor, and 200 yuan for other expenses, totaling 2,530 yuan. This results in a profit of 5,870 yuan per mu.
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