Analysis of Eight Major Misconceptions in Soilless Culture of Ricefield Eel
Originally published in Scientific Fish Farming, 2009, Issue 11
The ricefield eel (Monopterus albus) is a favored delicacy, and its market price has consistently remained high. Soilless culture has low pond construction costs, makes eel harvesting easy, allows high stocking density, yields high output and delivers good returns; it is suitable for both indoor and outdoor farming and has promising prospects for development. However, in practice, some one-sided and even erroneous views have spread widely. Misconceptions exist mainly in the following eight aspects, which have constrained the healthy development of the ricefield eel farming industry.
1. Using only aquatic plants as fish nests
Through long-term natural selection, the ricefield eel has developed the habit of dwelling in burrows, the significance of which lies in evading predators and avoiding the onslaught of high temperatures and severe cold. In practice, the dense root systems of aquatic plants are generally used as fish nests. However, using only aquatic plants as nests easily causes the eels to cluster and become entangled, resulting in excessive local density; moreover, because water temperature fluctuates considerably, eels sheltering among aquatic plants are prone to colds. Combining discarded tires with black plastic bags to provide nests for soilless eel culture keeps the ambient temperature stable, facilitates the eels' free entry and exit, and also overcomes the drawback of nests such as PVC pipes, tiles, stones and bamboo tubes, which are inconvenient for the eels to enter and leave freely.
2. Stocking fry and fingerlings too late
At present, soilless ricefield eel culture follows the practice of cage culture for stocking fry and fingerlings, that is, concentrating stocking at the end of June to early July, which results in higher fry prices and a shorter culture season (about 3 months). In cage culture, the eels inhabit the aquatic plant layer, so stocking must wait until the water temperature has stabilized (that is, late June to early July). However, in soilless culture in cement ponds, the eels inhabit the tires at the bottom of the water, where the water temperature is relatively stable, so fry and fingerlings can absolutely be stocked around Qingming.
3. Disinfecting fry and fingerlings with salt water and the like
At present, artificial breeding technology for ricefield eels in our country has not yet reached the level of mass-producing and supplying stock for commercial farming, so many farmers buy wild eel fry from the market and mostly disinfect them with salt water, potassium permanganate, iodine preparations and the like, which causes the eels to shed large amounts of mucus and affects their survival rate. The scales, gills and fins of the ricefield eel are degenerate, and its outer barrier is mucus and skin; the mucus contains large amounts of lysozyme and has extremely strong resistance to bacterial infectious diseases. Therefore, when disinfecting fry and fingerlings, it is best to choose drugs that cause little irritation to eel mucus, such as gentamicin, chlortetracycline and Duoyishan No. 1, so as to raise the survival rate of the eels and prompt them to start feeding earlier.
4. Sizes cannot be mixed
In practice, many farmers believe that large and small ricefield eels cannot be mixed, and during sorting they carefully separate the large eels from the small ones; after a period of culture, because growth is uneven, they must again divide the eels into ponds by size, which is troublesome and, because of the repeated handling, affects the eels' survival rate. It is true that ricefield eels do prey on smaller ones, but not as seriously as some references suggest. Regardless of hunger, eels only exhibit predation on small eels after individual differences reach a certain level. According to research, the frequency of wild juvenile eels swallowing fry is 2.54%, with a minimum individual length of 192 mm; the frequency of adult eels swallowing fry and juveniles is 11.43% [1]. It can be seen that as individual differences increase, the frequency of eels preying on small eels rises. Only after the body weight ratio reaches 25:1 do eels exhibit cannibalism, and large eels do not affect the growth of small eels at all [2]. Therefore, when sorting, it is only necessary to pick out the larger eels; there is no need to divide them into many groups for culture according to size differences.
5. Pond water too deep
The ricefield eel is the only fish that can drown. Because its gills are severely degenerate, the eel mainly breathes air through its mouth and nostrils, and the amount of oxygen it obtains this way accounts for about 2/3 of the oxygen the eel requires [3]. Therefore, the water in eel culture ponds should not be too deep; if the eels frequently swim to the surface to breathe, affecting normal growth, it is mostly caused by the pond water being too deep. Of course, the water should not be too shallow either, otherwise the temperature will fluctuate too much. Generally, the water depth for pond-cultured ricefield eels should be around 20-30 cm.
6. Feeding floating feed multiple times a day
The ricefield eel is an omnivorous animal that is mainly carnivorous; it feeds by swallowing, mainly by biting with its mouth, and a crisp sound can be heard when the water is clear, with feeding mainly taking place underwater. The eel has a certain fixed preference for bait, and if the type of bait is suddenly changed, the eel will refuse to eat, affecting normal life and growth. If a change of bait is truly needed, the proportion of the original bait should be gradually reduced while increasing the proportion of the new bait to make the adjustment. The first defecation of the ricefield eel occurs 24 hours after feeding, so feeding once a day is sufficient to meet the eel's feeding needs [4]. Research has shown that body weight has no significant effect on the daily feeding rhythm of the ricefield eel; the order of the eel's feeding proportions in different time periods, from high to low, is: 20:00-22:00, 22:00-24:00, 0:00-20:00, with almost no feeding activity between 4:00 and 16:00 [5]. In outdoor artificial culture of ricefield eels, the eels can be trained to form the habit of feeding at fixed times and fixed locations, feeding once a day, with the feeding amount such that the eels can consume about 80% of it within 2 hours. Practice has proven that the proportion of sinking feed consumed by ricefield eels is significantly higher than that of floating feed, and since floating extruded feed is more expensive, good sinking feed can absolutely be used for feeding.
7. Light too dim
Being active by night and hiding by day is another dwelling characteristic of the ricefield eel; this characteristic is likewise conducive to evading predators, but it is also a need for the body's own protection. Prolonged unshaded light exposure (more than 10 days) reduces the barrier function of the eel's body surface and its immunity, and the disease incidence rises rapidly, which indicates that the ultraviolet component of sunlight is harmful to the ricefield eel. Therefore, many farmers set up shade material over the ponds, but this easily makes the light too dim. Repeated experiments have shown that low-light conditions (less than 250 lx) and a still-water environment inhibit the natural reproduction of the ricefield eel [6], and the aquatic plants do not grow well, affecting the purification of water quality. Therefore, ricefield eel culture should maintain a certain amount of light, the aquatic plants should cover about 2/3 of the water surface, and in summer there should be shading, with diffused light being best.
8. Water temperature must not exceed
When the water temperature is below 8°C, the ricefield eel does not eat; below 5°C it begins hibernation; from 8-15°C its body weight does not increase. The eel generally grows suitably at 15-32°C, with the optimal growth temperature being 25-32°C; when the water temperature is above 36°C it retreats into its burrow to pass the summer, and at 40°C it dies. At present in practice, when the water temperature exceeds 30°C, cooling measures are immediately taken to avoid the effects of high temperature. Experiments have proven that the ricefield eel has a relatively high tolerance for high temperature and still shows a high appetite at 35°C [7], which is related to the eel's origin in the high-temperature Indian plains or the Sino-Indian foothills. Therefore, in artificial ricefield eel culture in Nanjing and areas to its south, the water temperature in summer should be maintained at around 30°C, so as to make full use of solar energy resources, increase the eel's feed intake and growth rate, and shorten the culture cycle.
References:
1. Yang Daiqin, Chen Fang, Li Daoxia, et al. A preliminary study on the feeding habits of the ricefield eel [J]. Acta Hydrobiologica Sinica, 1997, 22(1): 25-30
2. Zhou Wenzong, Li Hongtao, Gao Hongli, Zhang Luo. Patterns of predation by the ricefield eel on several small aquatic animals [J]. Jiangsu Journal of Agricultural Sciences, 2007, 23(2): 119-123
3. Wu H W and Kong C C.On the accessory respiratory organs of Monotperus [J].Sinensia，1940，11：59～67
4. Zhou Wenzong, Zhang Luo, Gao Hongli, Li Hongtao. A preliminary study on the defecation activity of the ricefield eel [J]. Acta Hydrobiologica Sinica, 2008, 32(3): 322-326
5. Zhou Wenzong, Zhang Luo, Gao Hongli, Li Hongtao. Effect of body weight on the feeding rhythm of the ricefield eel [J]. Water Conservancy Fisheries, 2007, 27(1): 20-21
6. Zhou Wenzong, Zhang Luo, Gao Hongli, Li Hongtao. Study on shallow-water soilless semi-artificial propagation of the ricefield eel [J]. Acta Agriculturae Universitatis Jiangxiensis, 2007, 29(1): 105-109, 123
7. Zhou Wenzong, Li Hongtao, Zhang Luo, Gao Hongli. Effects of body weight and temperature on the maximum feeding rate of the ricefield eel [J]. Acta Hydrobiologica Sinica, 2008, 32(6): 889-893