Key Management Techniques for Raising Rice Field Eels in Cement Ponds

1. Construction Techniques for Rice Field Eel Ponds
   
Eel ponds can be built with brick and stone, with the pond walls, bottom and corners sealed with cement mortar or lime-sand-clay mixture; in areas where lime is plentiful, lime-sand-clay mixture can also be used to build the ponds. At present, there are three common types of eel ponds.
① Gravity-flow water-exchange eel pond: Its feature is that the bottom of the 80 cm deep eel pond is higher than the drainage ditch; the pond is rectangular, with an area of 3X 4 or 3X 5. 
  ② Combined habitat and feeding pond: Its feature is that the habitat and feeding areas are separated, 
  ③ Simple pond: In some places the pond wall foundation is slightly deeper, and the pond bottom does not need to be paved with bricks or stones, which can reduce pond construction costs. 

   Regardless of the type of eel pond, there must be one water inlet, one drainage culvert and one overflow outlet. The water inlet is best connected to a trough so that water falls from the trough into the pond; when filling with water, this can on one hand aerate and increase oxygen. On the other hand, it can prevent rice field eels from escaping along the water inlet. On the side of the pond opposite the water inlet, open a drainage culvert, and open an overflow outlet at the pond surface; fish-blocking facilities must be installed at every water opening. One third of the pond surface can also be used to grow aquatic plants such as water hyacinth, water lettuce and duckweed, and pumpkins and other crops can be planted beside the pond, with frames built over the pond for shade, creating good ecological conditions for the growth of the eels. Before stocking, the eel pond should be disinfected with quicklime; generally, after draining the water, use 50—60 kg of quicklime per mu of pond area, and only after about half a month following disinfection can eel fry be stocked. For household rice field eel farming, small ponds and other scattered water surfaces in front of and behind the house can also be used and converted into eel ponds. The area can be large or small, but there must be a water source so that water can be added or exchanged manually during drought. A pond depth of 0.7—1 m is suitable. For newly built fish ponds, it is best to choose a clean place that is sheltered from the wind, faces the sun, is near the residence and close to a water source, with an area of about 2—10 sq m, and 4—5 sq m is best.  No matter which type of fish pond, it must not leak water. 

   2. Feeding and Management Techniques 

   ① Sufficient water source and good water quality: Rice field eels require fertile, clean water with sufficient dissolved oxygen for growth. Rice field eels are animals that prefer shallow water and warm conditions, so in early spring the water level should be controlled at about 6 cm to help the water temperature rise. The body surface of rice field eels has a lot of mucus. If the storage density is high and the water is not changed in time, the mucus will accumulate more and more. When this mucus decomposes, it consumes a large amount of dissolved oxygen and produces heat, causing the water temperature to rise significantly and resulting in the death of the eels. Therefore, water exchange must be done well to keep the water fresh, increase the appetite of the rice field eels and strengthen feed conversion. Water exchange is an effective way to remove pond waste, increase dissolved oxygen and keep the water fresh. Feeding should not be too much; excess feed will rot and stink, causing water quality to deteriorate. Generally, 1 to 2 days after feeding, new water needs to be added or exchanged once. During the growing period, maintain a water depth of 10 to 15 cm, and adjust the water depth according to weather changes. 
② Pay attention to changes in water temperature: Around Qingming, when the water temperature is generally stable above 10°C, preparatory work can begin. First clean the environment around the breeding pond, then carry out chemical disinfection. The method is to make a saturated lime solution of 50 g/L with quicklime, filter out the clear liquid, add 10 g/L of caustic soda and stir evenly to make a strong alkaline disinfectant, then spray this liquid to disinfect the area around the breeding pond. Finally, remove the overwintering covering on the surface of the eel pond, and immediately spray the disinfectant on the mud surface and the inner walls of the pond. After completing the above work, check the overwintering condition of the rice field eels and whether the facilities of the eel pond are intact. At the same time, the mud ridges in the pond should also be repaired and planted with aquatic plants such as arrowhead, water chestnut and taro. In spring, temperatures are relatively low, and sometimes a “late spring cold” occurs. Therefore, heat preservation work must be done well. At first, the plastic film used for overwintering can be temporarily borrowed for covering; at noon on a windless, sunny day, uncover the film to let it sun for a few hours, then cover it again in the afternoon to keep warm. If conditions permit, electric lights or other heating facilities can be used. In summer, in addition to planting aquatic plants in the pond, some melons and beans can also be planted beside the pond, with frames built for shade. In winter, when temperatures are low, keep out the cold, drain the water, and cover with a layer of rice straw to maintain pond temperature and soil moisture. Prevent the eels from freezing to death, but do not walk around at will in the pond or pile heavy objects on it, so as to avoid compacting the underground channels, causing the ventilation holes to become blocked and leading to suffocation and death of the rice field eels. 
③ Strengthen patrol and observation: Always pay attention to observing changes in water quality, water temperature and pond water depth. During thunderstorms and heavy rain, strengthen supervision to prevent floods from bursting the pond or the eels climbing out of the pond. 
④ Prevent poisoning and damage: Prevent toxic substances from entering the eel pond. Geese and ducks must not be released in the pond, so as to avoid swallowing young eels and reducing profits. 
⑤ Properly maintain water depth: Generally, a depth of 10—15 cm is suitable; in high-temperature seasons, appropriately deepen the water level, but it must not exceed 20 cm. Because rice field eels are relatively sensitive to water temperature differences, once the temperature difference during water exchange exceeds 3°C, the rice field eels may catch a cold and gradually die. Therefore, when changing water, exchange with water of the same temperature as much as possible. A common measure is to build an additional storage pond, and exchange water after it has been exposed to the sun. The number of water exchanges should be flexibly controlled according to the stocking density of the rice field eels, feeding conditions, air temperature and so on, and the time is generally best at noon on a sunny day. When the air temperature is relatively low, do not change the water. When the air temperature rises to 15°C, change the water once a week, each time exchanging 1/4 of the total water volume; when the air temperature rises to about 20°C, change the water once every 5 days, each time exchanging 1/30. New water should be added frequently. The specific interval between water exchanges is once every seven days in spring and autumn, and once every three days in summer. The number of water exchanges must also depend on the water source, the number of rice field eels and the water quality. After winter begins, drain all the pond water and only keep the pond soil moist. Cover the pond soil with a layer of 6-8 cun of rice straw or other weeds to prevent the pond water from freezing.  Because there is no water in the pond, especially prevent cats, dogs and rats from entering the pond and swallowing the eels.