Key Technical Points for High-Density Rice Field Eel Farming in Small Ponds
Since the rice field eel (Monopterus albus) is a predominantly carnivorous fish, it has the habit of cannibalism (the big eating the small) when feed is severely insufficient. Therefore, to carry out high-density intensive rice field eel farming, it is necessary to use cement ponds, earthen ponds, or net cages with an individual area of less than 30 sq m to grade and raise the eels by size. Practice has proven that raising rice field eels by the small-pond high-density method has many advantages such as convenient management, high survival rate, fast growth, small land occupation, and very significant benefit per unit area. The key techniques are introduced below.
Design and Construction of Small Ponds
Currently, the farming methods commonly used for rice field eel farming include net cages, cement ponds, and earthen ponds. Which specific method to use can be decided according to local conditions and the farmer's own specific circumstances: those with fish ponds or reservoirs can use them for net-cage rice field eel farming; those with rice fields that have good water-storage conditions can slightly modify them into earthen ponds for raising rice field eels; those using dry land or rice fields with poor water-storage conditions to raise rice field eels need to build cement ponds.
1. Installation of net cages: The net cages used for farming rice field eels are generally made of durable polyethylene mesh cloth, costing generally 6-10 yuan per sq m, and can be used for about 3 years. The shape of the net cage is best rectangular, with a bottom area of 6-30 sq m, a depth of 1-1.5 m, and it can be sewn firmly with nylon thread. For setting net cages in ponds with a water depth of less than 1.5 m, the cages can be fixed by driving stakes directly into the pond bottom; for setting net cages in deep-water ponds or reservoirs, floating net cages can be set up using waste oil drums, tires, and the like.
2. Construction of earthen ponds: Building an earthen pond is actually just converting a rice field into small earthen ponds 3 m wide and no more than 10 m long by piling up mud ridges. Generally, converting 1 mu of rice field requires an investment of only about 3000 yuan, making it currently the most economical way to raise eels. Each pond is required to have a water-storage depth of more than 50 cm, and a drainage ditch should be set between every two rows of ponds so that the water of a single small pond can be drained separately when needed. The perimeter of each earthen pond is protected against escape with polyethylene mesh cloth: use polyethylene mesh cloth more than 1 m wide, bury it 20-30 cm under the soil, sew the joints well with nylon thread, and support the pond rim with bamboo poles so that the mesh walls stand upright all around, preventing the rice field eels from escaping.
3. Construction of cement ponds: The one-time investment in building cement ponds is relatively large, generally 30-50 yuan per sq m, but the service life is long, management is more convenient, and water-saving performance is good, so it is also a method that eel farmers are happy to adopt. The size of cement ponds is generally 10-30 sq m, with a pond depth of 60-80 cm. The pond rim is laid with protruding bricks, and the four corners are made curved to prevent the rice field eels from escaping. The pond bottom is laid with concrete so that the bottom slopes slightly toward the water outlet side. On the lower side of the pond bottom, an overflow drain pipe is installed: take two plastic pipe elbows with a diameter of more than 5 cm, connect them with a section of pipe longer than the thickness of the pond wall, and bury it at the water outlet position so that one elbow is inside the pond and one is outside. The inner elbow opening is level with the lowest point of the pond bottom, and a pipe of matching size is inserted into the outer elbow; the length of the pipe depends on the required pond water depth, generally 30-40 cm. Over the inner elbow opening, a small plastic basket with small holes is placed upside down and pressed with 1-2 bricks to prevent the rice field eels from burrowing into the overflow drain pipe and escaping. Between every two rows of cement ponds, a corresponding drainage ditch should be set up, and a blocking net should be installed at the lowest point of each ditch to prevent the rice field eels from escaping should they get out of the eel pond.
Treatment of Newly Built Eel Ponds (Cages) and Laying and Disinfection of Aquatic Plants
1. Dealkalization treatment of cement ponds: Newly built cement ponds should be filled with water and soaked for more than 7 days, then the pond walls and bottom should be scrubbed with a brush to prevent the floating ash of the cement from harming the rice field eels. If urgent use is needed, an appropriate amount of acetic acid or other acid solution can be added to the water to shorten the soaking time.
2. Soaking and softening of mesh cloth: Newly installed net cages should be placed in the water in advance so that some algae grow on their surface and make them smooth, so as to avoid scratching the eel body. Earthen ponds that use mesh cloth for escape prevention should also be filled to the deepest water level for soaking.
3. Laying of aquatic plants: After the cement pond has been soaked and washed, the pond water should be drained and new water refilled, then aquatic plants can be laid in the pond. For earthen ponds, aquatic plants can be laid directly. The method of laying aquatic plants in cement ponds and earthen ponds is as follows: select fresh water hyacinth, and any dead old plants should be removed to prevent them from rotting after being placed in the pond and polluting the pond water. Lay the selected aquatic plants tightly into the pond, block them around with bamboo poles, and leave no aquatic plants within 40 cm of the pond wall, so as to prevent aquatic plants from growing at the pond edge and making it easier for the rice field eels to escape. In net cages, either alligator weed or water hyacinth can be laid. If the aquatic plants do not grow luxuriantly enough, some decomposed human and livestock manure water can first be splashed on to cultivate them.
4. Disinfection of the eel pond and aquatic plants: After the aquatic plants have been laid, drugs should be used to disinfect and deworm the eel pond and aquatic plants. The method is: for each cubic meter of water, take 2 ml of "Shanbao No. 2" or 30 g of quicklime, mix with water and splash over the whole pond, including onto the aquatic plants. The amount of dilution water should be as large as possible to prevent burning the aquatic plants. At the same time, for each cubic meter of pond water, use 5 g of "Shanbao No. 1" or crystalline trichlorfon mixed with water and splash it on to kill parasites in the water and on the aquatic plants. Two days after completing the disinfection and deworming work, the eels can be stocked into the pond.
Selection and Stocking of Eel Fry
The sources of rice field eel fry are divided into two methods: self-breeding and self-raising, and purchasing wild rice field eels for farming. Here the focus is on introducing the purchase of wild rice field eels for farming. Purchasing wild rice field eels for artificial farming is currently a method commonly adopted by the vast majority of eel farmers. Generally, they are purchased and farmed from April to September, and caught and sold on the market in winter or around the Spring Festival, with a weight gain of 1-5 times. Generally, the summer purchase price is below 20 yuan/kg, and the winter selling price is above 40 yuan/kg, so the farming benefit is very considerable.
Generally, the sources of rice field eels used for farming include: cage catching, electric catching, hand catching, etc.; for rice field eels caught by angling or clamping, they should be selected as appropriate according to the degree of injury; rice field eels caught by drugs must absolutely not be used. The methods of purchasing rice field eels are generally self-catching and self-raising, ordering from familiar eel catchers, or purchasing from township markets; rice field eels that have been stored at high density for a long time and those from large farmers' markets cannot be used for farming. When purchasing at the market, check whether the rice field eels have red or black gills, red and swollen everted anus, obvious external injuries, and other phenomena. If one household's rice field eels have these phenomena, do not purchase them; if you are not very good at identifying them when purchasing for the first time, you can look at a few more rice field eels and compare their gill color and anus, and it will not be difficult to tell. The rice field eels purchased for farming are best at 30-80 g per fish; too large ones cost more and grow slowly after reaching 300 g, while too small ones are relatively fragile and have a high mortality rate after being injured by catching. The purchased rice field eels should immediately be added to pond water and aquatic plants no less than 4 times the weight of the eels, and immediately transported for stocking.
The stocking density of eel fry should be increased or decreased as appropriate depending on the intended farming duration, generally 1-3 kg per sq m. On the evening of the day the eels are stocked into the pond, feeding should begin, using chopped earthworms, pig liver, freshwater mussel meat, fish meat, etc. (if maggots are available, freshly scalded live maggots can also be used), and at the same time, for each 1 kg of fresh material, add the following drugs: 10 g of electrolytic multivitamin, 3 g of Shanbao New No. 4, and 5 g of Shanbao No. 5. Use a special binder or a small amount of wheat flour to attach the drugs to the feed. The feeding amount is 0.5% of the body weight of the rice field eels. The feed can be placed directly on the aquatic plants; in cement ponds, the feed can also be placed directly in areas of the pond without aquatic plants. As many feeding points as possible should be set up. On the morning of the next day, for each cubic meter of pond water, use 5 g of Shanbao New No. 5 per 1 cubic meter of water, mix with water and splash over the whole pond (cage). The above medication method should be carried out continuously for 3 days.
Feed Training and Daily Management of Rice Field Eels
1. Feed training of rice field eels: After the rice field eels are stocked into the pond, we should often observe their feeding situation. For ponds (cages) with good feeding, the feeding amount should be gradually increased. When the feed intake reaches more than 2% of the body weight of the rice field eels (fresh material weight), compound feed should be gradually added to the feed, with the added amount gradually increasing, until the feeding amount is 5-10% of the body weight of the rice field eels. If there is leftover feed, it should be removed every morning.
2. Management of newly stocked rice field eels: Normal rice field eels should move freely in the aquatic plants and will not climb onto the grass for a long time. If rice field eels are found climbing onto the grass or rolling in the water and showing other abnormal behavior, they should be picked out in time.
3. Cooling management: For pond-raised rice field eels, in the high-temperature season, a shade net should be set up over the pond to reduce the water temperature on the one hand and also reduce the light intensity on the other. In high-temperature weather, the pond water should be deepened to more than 40 cm, and the pond water temperature should be controlled below 32°C as much as possible.
Harvesting of Rice Field Eels
For cement ponds and net cages, harvesting rice field eels is very convenient; after removing the grass, they can be caught directly. For earthen ponds, the method of piling grass to catch them should be used. The method is: leave only about 1 sq m of aquatic plants in a pond, and deepen the pond water to more than 50 cm. At this time, the rice field eels will generally gather in the aquatic plants. Every morning, use a dip net larger than 1 sq m to move upward from below the aquatic plants until the surrounding area emerges from the water, pick out the aquatic plants, and all the rice field eels will be in the net. In this way, 3 times can catch all the rice field eels in the pond. This method is only effective during the feeding season of the rice field eels. If they need to be sold in winter, they should be caught in advance before feeding stops and temporarily raised in another pond (cage).