Fattening techniques of Scylla serrata

Scylla serrata Forskal is abbreviated as Scylla serrata Forskal. The sexually mature female crab has higher economic value in mariculture because of the "red paste" on both sides of the carapace. Therefore, fattening green crabs is a good way to improve the quality of green crabs listed and increase the economic income of farmers. The fattening of green crabs refers to the process of cultivating female crabs (mated) of about 200g and thin water crabs into mature ovaries and carapace crabs and meat crabs after about 20-40 days of intensive breeding. The whole cultivation process is as follows: Individual Selection of Green Crabs: During the whole fattening process of green crabs, female crabs should be accurately identified and classified according to whether they have mated and gonad maturity. 1. Unmated female crabs, also known as white crabs. Generally small individuals, abdomen was gray black. The crab can only be cultivated as a meat crab and cannot be directly cultivated into a crab. 2, the first mating female crab, generally large individual, in the light observation, can be found in the head of the chest side edge of a semilunar ovarian gland, this crab after 30-40 days of cultivation, can become a crab. 3. Flower crab: The female crab of the first mating, after 15-20 days of culture, the ovary gradually develops and becomes a "flower crab." Flower crab ovary has not yet expanded to the edge of the carapace, and then after 15-20 days of feeding, it becomes a crab. 4. The individual bred must be a seedling with strong physique and sound appendages. For female crabs, parasitic crab slaves must be removed. Because parasitism has crab slave female crab, its ovarian development can be affected greatly, cannot breed crab. Fattening pool selection: The fattening pond should be selected near the middle tide line in the inner bay to avoid wind, so as to control the water level according to production needs, and require fresh water sources to adjust the water quality at any time; there is no sewage pollution around. We generally use "Tian" shaped pool, which is composed of four small pools connected into "Tian" word, with a total area of 1-5 mu. In the center of the pool, a square pool with a width of 1.5m is set up, and a sluice is set on each side of the pool to connect with four crab pools, and a ditch is prepared to inject water through the pool, so that the crab can automatically go upstream into the central pool for catching. Sediment is preferred. stocking: The most suitable water temperature for green crab fattening is between 18-25 degrees Celsius. At this time, green crab has large food intake, vigorous metabolism and rapid growth. Therefore, green crab fattening culture time is generally from August to May of the following year. The stocking density should be determined according to the season, feed and other conditions, and the stocking density should be 3-5/m3. It is advisable to adopt separate breeding of male and female crabs to avoid male crabs interfering with female crab growth. Daily management techniques: (i) Feeding. Green crab is a benthic crustacean living in the intertidal zone. It has the foraging habit of lying in the daytime and coming out at night. Its feeding habit is mainly carnivorous and omnivorous. In the production, to feed some fresh, seawater low-value shellfish mainly, such as blue clams, short-toothed clams, snails, mussels, etc.; before feeding, generally to crush the shells before feeding. In addition, according to needs, can be fed a small amount of miscellaneous fish, shrimp, crab, and some comprehensive nutrition of the feed. Generally speaking, the daily feeding amount in April-May and August-September should be 10% of the total weight of green crabs, and gradually reduced to about 5% from late September to March of the following year. Generally speaking, the feeding amount can be appropriately increased during the flood season, and vice versa during the flood season; the feeding amount should be reduced when the water quality deteriorates in hot and humid weather. (b) Water quality control. Taking necessary measures to control water quality is an important content of fattening management technology of green crab, especially in the place where water quality conditions are poor and water intake and drainage are not convenient. 1. Changes in water salinity. Different salinity of seawater has great influence on the growth of green crab. Generally, the optimum salinity is between 13-26‰, but normal culture can be carried out in the estuary sea area with salinity of 6-8‰. The adaptability of green crab to salinity mutation is relatively poor. A large amount of fresh water rushes into crab pond during rainstorm, causing sudden drop of salinity in pond water, which is easy to cause "white awn disease" of green crab. At this time, the upper fresh water should be drained or the method of hanging salt should be adopted to make up; and after the rain, the whole pool should be sprinkled with 0.2ppm dibromohydantoin to disinfect the water body to prevent diseases. 2. Control the change of water temperature. The most suitable water temperature for green crab fattening is between 18-25 degrees Celsius. When the water temperature is as high as 33-35 degrees, the green crab will appear obvious discomfort. Crawling around the embankment, trying to escape. At this time, the water level should be maintained at about 1.3-1.5 meters, and if conditions permit, it should be maintained in a state of micro-flowing water. When the temperature is lower than 7 degrees, the crab completely stops feeding and enters hibernation to spend the bad environment. 3. Changes of dissolved oxygen in water. After many years of observation, when the dissolved oxygen in water is greater than 2mg/l, the green crab lives normally and grows well; when the dissolved oxygen drops to 1mg/l, the green crab is slow to respond, does not eat, and appears "floating head" or even death. When anoxia occurs in the pond, anaerobic decomposition of organic matter at the bottom of the pond will occur, resulting in excessive accumulation of non-ion ammonia (NH3-N), which will be toxic to green crabs. When the crab molts, the requirements for dissolved oxygen are slightly higher than usual, so it is necessary to change the water frequently, usually once every 3-4 days to keep the water clean and dissolved oxygen sufficient. 4. Changes in water quality factors. pH value is a comprehensive indicator of the state of water quality. Generally speaking, the pH value of the crab pond should be kept between 8.0 and 8.5. In addition, the transparency of the pool water is also an important factor affecting the success of fattening. According to our experiments, the transparency of the pool water should be maintained at 20-25cm. Within this range, green crabs can move normally, and the chances of killing each other are relatively small. When the transparency drops to 15cm, if no measures are taken in time, it is easy to cause water quality mutation and cause the death of green crabs. 5. Tour the pond. During the breeding period, we must patrol the pond frequently to observe the water color, water level and activities of green crabs in the pond. Since green crabs have the habit of escaping from harsh environments, when water quality changes or oxygen in the water, green crabs will climb out of the water and escape across the dike, so escape prevention is very important. When this kind of situation is found, corresponding remedial measures should be taken immediately, such as starting the aerator, changing the water, sprinkling 0.40ppm of Yishuibao in the whole pool to improve the water quality, etc. 6. Prevention of diseases. During the fattening period of green crabs, the weather and temperature generally change greatly, so the water quality changes greatly, and green crabs are prone to diseases. Farmers must regularly prevent disease. Biological enzyme active additive and immune polysaccharide are added into feed to improve immunity of organism, and disinfectant such as Qiangke 101, Qiangke 202 and dibromohydantoin are used to improve water quality condition and maintain good environment of water body. When encountering diseases, correct diagnosis and active treatment should be adopted. (For detailed disease treatment plan, see China Fisheries, Issue 9, 2002) 7. Harvest. Green crab seedlings from stocking, if managed properly, 20 days can be harvested on the market, can be harvested crab must be "full of meat". For female crabs, their mature ovaries have been filled with the teeth of the head chest, at this time the cream accounts for 80-90%, its fishing method is similar to that of ordinary crabs, and it should be often drenched in water during bundling transportation to keep its gills moist and ensure that live crabs are listed.