Detailed Explanation of Static-Water Soilless Culture Techniques for Rice Field Eels

      In the wild, rice field eels often inhabit shallow muddy water because mud provides stable warm conditions and a dark living environment. In production, using bamboo tubes, bricks and tiles, aquatic plants, etc. as fish nests to replace mud can satisfy the living conditions of rice field eels, allowing soilless culture. Compared with soil-based culture, soilless culture of rice field eels has the advantages of lower pond construction cost, convenient observation and management, easy training to accept compound feed, and benefits for preventing and controlling diseases and pests, with relatively higher returns. At present, soilless culture basically adopts the flowing-water culture model, but in places where water resources are scarce, the flowing-water soilless culture model is difficult to promote. Static-water soilless culture of rice field eels, however, is easier to promote than flowing-water soilless culture. Static-water soilless culture saves water and labor, requires little investment, takes effect quickly, and has a wide range of adaptability. Generally, after a production cycle of 6 to 7 months, the eels can gain 3 to 5 times their weight, and the net income per square meter of eel pond can reach 100 to 150 yuan. The key techniques of static-water soilless culture of rice field eels are briefly introduced below.
1. Planting of Aquatic Plants
Rational placement of aquatic plants can purify water quality, reduce the number of water changes in the eel pond to 1 to 2 times per month, and can also play roles in preventing heat and lowering temperature, reducing stress reactions, providing fish nests, and preventing and controlling diseases and pests. Common aquatic plants include water peanut, water hyacinth, water lettuce, and fine azolla. Aquatic plants should be reasonably matched in proportion according to different seasons. In summer, water hyacinth and water lettuce are dominant; in spring and autumn, water peanut and fine azolla are dominant; in winter, no aquatic plants should be left to prevent rice field eels from sheltering under the plants and freezing. Generally, aquatic plants are placed 15 days before stocking the eel fry. Before placement, the aquatic plants should be soaked in a 0.01% potassium permanganate solution for half an hour for disinfection. The planting area of aquatic plants should not exceed 2/3 of the total pond area, and at least 1/3 of the pond area should be left free for setting up feeding platforms and for the eels' activities. In daily management, excess aquatic plants should be promptly removed or overly long aquatic plants should be cut. In conjunction with pond disinfection, 0.1×0.000001 quicklime should be splashed on the plants to prevent infection by pathogens on the aquatic plants.
2. Setting Up Fish Nests
Various tubes, bamboo tubes, bricks and tiles, waste tires, aquatic plants, loofah sponges, palm sheets, polyethylene mesh, etc. can all be used as fish nests. In production, waste bicycle tires disinfected with potassium permanganate solution are generally used as eel nests with relatively good results. The tires are placed under the aquatic plants, and 5 to 6 tires can be placed in each small pond. Bamboo tubes can also be used, with two bamboo tubes as one row, and 3 to 5 rows set up per pond, with a spacing of about 0.3 m between rows. Bricks are placed under each row of bamboo tubes so that there is a larger space below the bamboo nests, facilitating water flow and waste discharge. To fix the bamboo nests, it is best to place bricks above their openings, which also serves to block light and provide concealment.
3. Heat Protection and Shading
Static-water soilless culture ponds for rice field eels are relatively small (15 to 20 sq m), with shallow water levels (15 to 20 cm). In summer, the water temperature can easily exceed 30°C, so shading and heat protection measures for the pond are essential. Generally, a shade net is set up on a frame above the pond, and climbing plants such as grapes, loofah, and pumpkin are planted along the pond edge. However, note that 10% to 30% of light should be allowed to reach the water surface. When the air temperature exceeds 30°C, the water level in the pond should be deepened, the density of rice field eels should be reduced, and new water should be slowly injected (using groundwater to regulate the pond water temperature).
4. Water Quality Regulation
4.1 Alternating Disinfection
Every 10 to 15 days, the water body should be splashed throughout the pond with 0.1×0.000001 quicklime or 1×0.000001 bleaching powder, used alternately. In the north, where water quality tends to be alkaline, bleaching powder should be used less frequently; in the south, where water quality tends to be acidic, quicklime can be used more frequently.
4.2 Maintaining a Suitable Water Level
Rice field eels need to frequently extend their heads above the water surface when eating and breathing. To reduce the consumption of the eels' physical strength, the water level should be shallow. However, if the water level is too shallow, the water temperature changes quickly, and the eels' activity space is small, which is extremely unfavorable for the growth and development of rice field eels. Therefore, the water level is generally 10 to 15 cm, and can be deepened to 25 cm when the temperature is high. If the aquatic plants grow luxuriantly, changing the water once a month is sufficient, and generally clean water should be added in time according to the evaporation rate.
4.3 Maintaining Biodiversity
The eel pond should be stocked with field snails, small miscellaneous fish, loaches, etc. to remove residual bait and regulate water quality. However, care should be taken that these organisms maintain a reasonable proportional relationship in quantity. Per square meter of eel pond, the stocking of loaches should not exceed 0.3 kg. Loaches should preferably be stocked after the rice field eels have been trained to accept compound feed, so as to give full play to the role of loaches in eating the feces of rice field eels. Stocking toads has a special effect on preventing the plum blossom spot disease unique to rice field eels, and generally 1 to 2 toads per small pond is sufficient. Per square meter of pond, the stocking of field snails should not exceed 0.25 kg. In addition, an appropriate amount of green algae, etc. can also be cultivated in the pond.
5. Ecological Prevention and Control of Diseases and Pests
5.1 Regulating Water Temperature
The suitable temperature for the growth of rice field eels is 15 to 28°C, and the optimal feeding temperature is 23 to 26°C. When the water temperature rises, the growth and reproduction of pathogens that prefer low temperatures are inhibited, which reduces the occurrence of some diseases, such as saprolegniasis and white spot disease. Note that the daily water temperature difference should not exceed 10°C, otherwise printing disease is very likely to occur. In addition, regulating the water temperature to the optimal level will promote feeding by rice field eels and enhance their physique.