The Yellow River's estuary is the primary source of eel larvae, and the successful cultivation of natural seedlings in Haipu has made eels a key species for breeding in the Yellow River Delta. From 1998 to 2000, we utilized old shrimp ponds in Dongying City’s shrimp farms, relying on naturally cultivated biological feed to temporarily raise sea-caught eel fry. This method proved successful for three consecutive years, generating significant economic returns. The technique was evaluated by experts from the Shandong Provincial Science and Technology Commission and recognized as among the best in the country. Below is a detailed introduction of the technology:
**I. Holding Technology**
1. **Pond Conditions**: We used pond No. 5 of team No. 1 at the shrimp farm in Dongying City, covering 50 mu. The rectangular pond features a central ditch with inlet and drainage gates. The depth ranges from 1 to 2 meters, with a muddy-sandy bottom that had not been dredged.
2. **Stocking Preparation**: Before stocking, the pond was drained and disinfected using quicklime at a rate of 75 kg per mu. After 48 hours, water was introduced from the confluence of the Guangli and Yihong Rivers. This brackish water has a salinity between 5‰ and 15‰, with fresh water beneath and seawater tides. As spring progresses, the water becomes rich in natural food sources such as silkworms, clams, prawns, and small green shrimp. Every March 10th, high-tide water is drawn in through a mesh cone-type net to capture larvae and eggs. A 20-mesh net is placed at the drainage gate to prevent escape. Two days after the first inflow, the water is drained and the pond is washed twice before gradually filling it to 80 cm to allow natural reproduction of food organisms.
3. **Seed Stocking**: By late March, naturally grown eel fry from the river mouth are collected. These seedlings range from 1.5 to 3.0 cm in size, and the seawater salinity at the river entrance is around 20‰. To ensure survival, the seedlings must be desalinated. Weak or injured individuals are removed, and the salinity is gradually adjusted to match the pond’s level. Seedlings should be stocked within 7 days to avoid size differences and competition for food.
4. **Key Technical Measures During Holding Period**
- **Water Quality Management**: The water environment should remain stable. Since the water comes from both the sea and freshwater, physical and chemical parameters fluctuate. For the first 15 days, only high-tide water is added, each time raising the water level by 10 cm to maintain a salinity above 10‰. Once the water reaches 1.8 meters, an 8-mesh nylon net is used to prevent fish from escaping. Water is changed every 10–20 cm to keep it fresh and support food growth.
- **Feeding**: Eel fry primarily feed on natural shrimp and prawns in the pond. Small pull nets are used periodically to check food availability. If insufficient, live shrimp can be introduced to supplement the diet.
**II. Key Points to Note**
Through a three-year trial, we found that improving eel fry survival requires attention to three main aspects: (1) selecting healthy, uniform seedlings and stocking them at the right time; (2) maintaining a stable water environment due to the variable nature of the water source; and (3) ensuring sufficient natural food in the holding pond.
**III. Advantages of the Holding Technology**
1. Eel fry raised in low-salinity conditions are easier to sell. Carp struggle to survive in high-salinity northern waters but thrive in brackish environments. In the Yellow River Delta, brackish water is easily created using fresh water, surface seepage, and underground brine, making this method promising for future development.
2. Utilizing existing shrimp ponds increases their efficiency, reduces breeding risks, and improves overall productivity. Eels can be raised until late May, after which shrimp such as Chinese shrimp or Penaeus vannamei can be stocked. These shrimp thrive in low-salinity conditions, and any weak or sick eels can help control disease outbreaks in shrimp populations.
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