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Utilizing the Nursery Workshop Dongcun Sansui Swimming Crab Technology
The Tripitaka crab, a rare and highly prized edible crab found along China's coastal regions, is cherished for its exceptional taste and rich nutritional value. Having live crabs on the New Year’s table is not only a symbol of prosperity but also a long-standing aspiration among many. To achieve off-season marketing and boost profitability, we successfully implemented a winter seawater nursery system in 2001 and 2002. This method allowed us to increase the yield of Dongchu Tripitaka crabs by 1.5 to 3 times. Below are the key technical measures we applied for reference.
**1. Preparations Before Winter Storage**
**1.1 Workshop and Pool Renovation**
We utilized idle and nursery workshops that were easy to drain and had good insulation. These were covered with black plastic sheeting to maintain warmth and block light. The winter storage pools were designed with a bottom area of 20–40 square meters and a depth of 120–150 cm. Each pool was equipped with 6–10 air stones, evenly distributed at the bottom for oxygenation. Pools were thoroughly cleaned and disinfected before use. A layer of fine sand (10–12 cm) was laid, washed, and soaked in a 10 ppm potassium permanganate solution for 25–30 minutes. The sand covered 3/5 to 4/5 of the pool floor, leaving a 1/5 to 2/5 gap near the outlet for bait placement and water exchange. Two days before introducing the crabs, the pools were filled with filtered water up to 50 cm.
**1.2 Selection of Crabs for Winter Storage**
Winter storage timing depended on weather and production conditions, typically between mid-October and early December when outdoor temperatures ranged from 8°C to 13°C. Crabs were either collected directly from fishing boats or selected based on specific criteria: strong body, active movement, intact appendages, clean surface, and good fullness. Only female crabs were stored.
**1.3 Transportation and Acclimation**
Crabs were carefully tied to prevent damage during transport. They were then placed in an indoor holding pool with 40–50 cm of water for 4–8 hours to acclimate. During this time, they were rinsed to remove dirt and debris, followed by a 20–30 minute bath in a 300 ppm formaldehyde solution. After washing, they were transferred to the winter storage pools at a density of 8–12 crabs per square meter.
**2. Daily Management During Winter Storage**
**2.1 Feeding**
Bait was provided once daily in the evening, primarily consisting of live feed like sandworms and small fish. The amount was 1–6% of the crab's body weight, adjusted according to water temperature. Bait was washed and disinfected before being placed in designated areas, avoiding direct contact with the sand.
**2.2 Water Changes**
Water levels were maintained at 100–120 cm. When the water temperature exceeded 8°C, water was changed every 3 days, replacing 70–80% of the total volume. For lower temperatures, changes occurred every 5 days, replacing 50–60%. Residual bait and dead crabs were removed during each change, and the temperature difference was kept within 0.5°C.
**2.3 Temperature Control**
The ideal water temperature range was 5–12°C. Insulation with straw curtains and occasional heating were used when temperatures dropped below 5°C. Ventilation was increased when temperatures rose above 12°C. Daily temperature fluctuations were kept within 2–3°C.
**2.4 Disinfection**
When water temperatures were above 8°C, disinfection was performed weekly using a 100 ppm formaldehyde solution for 20–30 minutes. If temperatures were below 8°C, disinfection occurred every 10–15 days.
**2.5 Additional Management**
During rainy or low-pressure weather, intermittent aeration was used to prevent hypoxia. Crabs were checked three times daily for activity and dead individuals. Water temperature was recorded twice a day, and all activities were documented meticulously.
**3. Out-of-Pool Timing**
Crabs were released based on market demand and production conditions. Typically, it was best to release them around or after the Spring Festival, when prices were high. If water temperatures remained above 13°C for more than two weeks, crabs should be discharged promptly to avoid spawning.
**4. Winter Storage Test Results**
In 2001 and 2002, we conducted trials using a 480 m² nursery facility. In 2001, the first batch was released in February and the second in March, with a survival rate of 87.25% and 85.20%, respectively. In 2002, the survival rates improved to 93.40% and 92.71%.
**5. Discussion**
Using idle nursery facilities for Tripitaka crabs not only improved resource utilization but also enabled off-season sales, significantly increasing profit margins. After two years, the post-storage price was 1.75 and 1.2 times higher than the original, showing great economic benefits. The technology is practical and easy to implement.
The trial results showed an average weight gain of 4.08% and 5.45% in 2001 and 2002, respectively. The 2001 survival rate was lower due to inadequate insulation. With proper management, we believe survival can reach over 95% and weight gain exceed 7%.
Comparisons between sea crabs and pond-reared crabs showed no significant difference in survival rates under similar conditions. Therefore, sub-pool storage requirements were not necessary.
Regarding stocking density, tests showed that up to 15 crabs per square meter had minimal impact on survival, and continuous aeration was unnecessary under normal conditions.
In 2001, male crabs were also tested, but the survival rate was low, possibly due to physiological differences, though the exact cause remains unclear.