In recent years, traditional crab farming methods have typically involved seeding crabs one at a time with uniform sizes. However, through practical application in the past few years, it has been discovered that using a split-seeding method in natural large water bodies can yield better economic returns. The following are the key findings from the test:
First, the water conditions: The area under study covers 158 mu and is a natural river channel located in the Shu region. It is connected to external water channels, offering stable water flow with an average depth of 2.1 meters and a maximum depth of 3.5 meters. Two-thirds of the area is shallow, and the bottom consists of black loam.
1. Aquatic plant cultivation: From August 2004 to May 2005, 21 tons of algae were transplanted. In early April 2005, 203 kilograms of bitter grass seeds were planted. Deep-water species such as goldfish algae were used, while shallow areas were covered with Vallisneria. The coverage reached 70-80%.
2. Snail introduction: Snails were abundant in the ponds, and from November 2004 to April 2005, 36 tons of snails were introduced, including those from the original pond and 500 kg of additional snails.
3. Pond cleaning: Due to the large size of the water body and its connection to other channels, it was not possible to use feral fish for control. In November 2004, 14 tons of lime were applied, followed by 6 tons of quicklime in February 2005, which helped purify the water, making it clear up to 1.5 meters with a pH of 7.5-8.5.
Second, seed stocking:
1. Seed sources and specifications: At the end of May 2004, Chinese mitten crabs (Eriocheir sinensis) were purchased from the Sheyang Nursery in Jiangsu Province and raised locally. This helped them adapt to the environment and improved survival rates.
2. Stocking process:
- In August 2004, 50,000 crabs with a size of 400-500 eggs per kilogram were introduced. In July 2005, 31,000 fingerlings with a size of 120-160 eggs per kilogram were added, totaling 81,000 eggs.
- Additionally, 100 kg of young shrimp, 2,500 carp, and 1,500 carp fry were introduced during different periods.
- Before stocking, all seedlings were disinfected using a 3-5% saline solution for 10-15 minutes to eliminate surface bacteria and parasites.
Third, feeding and management:
1. Feeding strategy: Early feeding started in mid-February, mainly using chilled fish with some formulated feed. From May to September, food was provided in sufficient amounts to ensure good feeding, with about 8% of the crab biomass being fed. Vallisneria served as an indicator of feeding levels—when there was enough food, fewer leaves were eaten; when food was scarce, more leaves were consumed, prompting an increase in feed. Twenty to thirty days before harvest, high-protein feed like chilled fish was increased to improve crab quality and market value. A total of 52,000 kg of feed was used throughout the year.
2. Escape prevention: Preventing crabs from escaping was a major focus. Double barriers were installed at the inlet and outlet, and cages were placed in two layers. Daily checks of nets and barriers were conducted, especially during rainy seasons when water levels could rise quickly. Nighttime patrols were maintained to detect and address any escapes promptly.
Fourth, timely fishing: Crab fishing began in late September and peaked in early October. The primary method used was cage trapping.
Fifth, economic benefits: A total of 8,714 kg of marketable crabs were harvested, with an average weight of 215 grams and a catch rate of 50.03%. Additional catches included 503 kg of carp, 8,493 kg of carp, 1,351 kg of green shrimp, and 2,487 kg of other fish. The total output value reached 955,000 yuan, with crabs contributing 802,000 yuan. Total expenses amounted to 3.8135 million yuan, resulting in a profit of 573.65 million yuan, or 3,631 yuan per mu.
Sixth, lessons learned: The water body is a natural river ditch within a reclaimed area, connected to the outside with five inlets and outlets. The largest exit is about 65 meters wide, and water levels are unstable. Stumps were used to secure blocking facilities, ensuring a relatively stable water environment, especially during the rainy and flood seasons. Regular patrols were essential. Additionally, catching fish in a timely manner when crabs matured helped prevent escape and improve the withdrawal rate.
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