Key Techniques for Cultivating Middle-aged Shrike Crab

Scylla serrata, commonly known as blue crab, is an important mariculture crab with fast growth, strong adaptability (wide temperature and salty), delicious meat taste, rich nutrition, and high economic value. . China's S. marginata has been breeding for more than 100 years and was first used for fattening and stocking. Since the middle and late 1980s, with the development of artificial breeding of blue crabs and the promotion of breeding techniques, the blue crab breeding industry is located in the southeast coastal provinces of China. It has been booming, especially in coastal areas such as Fujian, Guangdong, Zhejiang, Guangxi, and Hainan, where the enthusiasm for blue crab breeding has increased, the area for aquaculture has been expanding, the aquaculture production has been continuously increased, and the aquaculture efficiency has been continuously improved, especially in the past ten years. The success of domestication and training has promoted the formation and development of the crab industry more powerfully. Green crab has become one of the important varieties of mariculture in China.

At present, the cultured crabs still rely mainly on natural sea catches. There are few artificially produced crabs, and the supply of seed is in short supply. The lack of seed types hinders the further development of the blue crab breeding industry. Blackshaw (1999) in their editor's book "Mud Crab Aquculture & Biologe" pointed out: "The production of blue crab aquaculture is still constrained by the supply of seedlings. The in-depth exploration of seedling raising techniques is the main task in the current practice of production of blue crabs." The current status of research on artificial breeding techniques of blue crabs is revealed.

Since the middle and late 1980s, the research team of the Qinghai crab team of Xiamen University has systematically studied its reproductive biology and artificial nursery techniques. It has made breakthroughs in artificial breeding and formed a complete artificial seedling raising process. In the mid-1990s, it was possible to carry out mass production and have already entered the stage of promotion and production. However, because China's crab farmers only raise young crabs in the culture of blue crabs, they do not stock large-eyed larvae. Large-eys larvae are the key stages in the transition from campylobactered to campycium. At this stage, great changes have taken place in the habits and habitats of large-eyed larvae, resulting in a very low metamorphosis rate of large-eyed larvae metamorphosing into juvenile crabs. This has hindered the large-scale promotion and industrialization of artificial breeding techniques Bottleneck.

The technical content of blue crab breeding is high, and it is difficult. Among them, it is difficult for big-eyed larvae to be cultivated into young crabs that can be reared for breeding. This hinders the further promotion of large-scale productive breeding of blue crabs. Therefore, Xiamen University also carried out the work of artificial breeding of young crabs and the reform of breeding models. After promoting the first and second stage of young crabs cultivated in the south of Anhui Province, the technology radiated from the south of Anhui Province to Guangdong, Guangxi, and Zhejiang, which made China's blue crab breeding technology a big step forward. Only young crabs (duck crabs) that can only be stocked in the seventh and eighth stages are improved to be able to stock their first and second young crabs. In addition, the cultivation technology of green crab is still not standardized, and breeding techniques are still relatively backward. To this end, key techniques for the middle cultivation of blue crab seedlings are summarized as follows to further promote the healthy, stable and sustainable development of the blue crab breeding industry.

The cultivation of blue crab seedlings refers to the process of intensive cultivation of big eye larvae into small crabs with a size of 1.2-1.8 cm. It is an important part of the productive cultivation of blue crabs, and whether artificial seedlings can be widely used for breeding. The key is to be able to achieve the required specifications of juvenile crab through intermediate cultivation, which can effectively increase the survival rate of the breeding, increase the economic benefits of the crab-raising industry, and achieve a qualitative leap for the crab industry. The cultivation of blue crabs should be based on the biological characteristics of blue crabs, focusing on solving the problems of the construction of intermediate cultivation ponds, the structure and setting of concealed objects, the density of seedlings, the type of feed and the amount of feed, the water quality regulation, and disease prevention and control. In order to improve the survival rate of the large-eyed larval mid-cultivation.

1. The construction of the intermediate cultivation pool and the establishment of concealed objects The intermediate cultivation pool can be a concrete pool or an earthen pond. It is close to the seedling production area. There are sufficient sources of fresh water, good water quality, no pollution, and convenient transportation. The general cement pool area is 50-100m2/mouth, the pool depth is about 1.5m, the pool wall is smooth, and the bottom is 3-5cm sand layer. The area of ​​the earthen pond is 350m2-660m2, and the shape is rectangular. The cultivation pond must have access to the water gate, pump, water shade and so on. At the same time, hidden objects such as oyster shells and palm pieces should be set in the cultivation pond for crabs to inhabit and reduce the killing of juvenile crabs and improve the survival rate of juvenile crabs.

2. Clear pool disinfection before clearing the seedlings, the cement pool can be cleaned with 100ppm bleach, and then soaked in 30ppm potassium permanganate solution for 30 minutes, and rinse with disinfectant clean seawater several times to be used; soil pool is in use before 7-10d Thoroughly sterilize 375-500 ppm of quick lime, and then lay a large "cage" of 8-16 mesh polyvinyl chloride mesh on the bottom of the pool and the pool. The well-sterilized pond is filled with filtered seawater that has been filtered through about 120 mesh, and the water level is 1m.

3. When crabs are stocked and stocked with crabs, they should pay attention to the temperature of the water in the pool and the seedling container. If the difference is too large, the temperature difference should be gradually reduced to about 1°C before they can be put into the pool. The stocking density can be appropriately adjusted according to the conditions of the breeding environment, the season, the expected discharge specification, and the bait conditions. The stocking density of macrolarvae in general cement ponds is 2500-3500/m2, and the stocking density in earthen ponds is 2000-2500/m2. The seedlings should be released in the morning to avoid the high temperature period. After 3-5 days after the crabs enter the pond, the salinity gradually fades from 30-35, adding 1/10 fresh water every day, and the salinity can be reduced to about 20 degrees.

4. Daily management is mainly to pay close attention to feed, water quality control, disease control, etc. In addition, we must pay attention to prevent escape. Under good cultivation conditions and proper management, when large-eyed larvae metamorphose into young crabs, the survival rate can reach 50%-60%, and the survival rate of juvenile crabs to commercial seedlings is 40%-50%. Doing day-to-day management work is of great significance.

(1) The feed is fed to the big eye larvae to the first stage of juvenile crabs before feeding Artemia larvae (preferably to strengthen nutrition, in particular to strengthen the Artemia fatty acids, vitamin C, E, etc.), living copepods And a small amount of frozen copepods; can then be frozen copepods, live small blue peony, fish, shrimp, shellfish clams and artificial compound feed mix, and with the development of development, the proportion of various baits should be properly adjusted, In the later period, it can be gradually transferred into special feeding formula for young crabs. In the early days, a small number of multiple feeding methods were used. After the third stage of development, the number of feedings can be gradually reduced, and the transition to feeding twice a day in the morning and evening, with feeding at 6 o'clock in the morning, and the amount of feeding for the total daily feeding amount of 30 %-40%, feeding at around 5:00 pm, feeding 60%-70% of total feeding, feeding about 10% of crab body weight, and according to the saturation of bigeye larvae and young crabs, The amount of water in the bottom of the pond, the remaining amount of the bottom of the pond, and the activity of the larvae and juvenile crabs were adjusted. Remove the bait and feces before feeding each morning.

(2) Water quality management: Water quality indicator requirements for intermediate cultivation: The salinity at the early stage of planting (3-5 days for stocking) is about 30, and then it gradually decreases until the salinity is about 20; the water temperature is maintained at about 30°C; the pH is 7.8 -8.6; dissolved oxygen at 4.5 mg/L; ammonia nitrogen below 0.02 mg/L; hydrogen sulfide less than 0.01 mg/L. In short, fresh crabs like to live in a fresh, oxygen-rich environment. Oxygen consumption in clam shells is greater and they are particularly sensitive to water quality. Therefore, crab ponds should maintain good water quality. Quality of water directly affects big eyes. For the growth and survival of larvae and juveniles, water quality management must be strengthened to monitor the water temperature, salinity, pH, and transparency. Water quality regulation should follow the following principles:

1After all the juveniles become juvenile crabs, besides replacing fresh seawater every day (not more than 1/3 each time), add fresh water slowly, changing the amount of water each time should not be too large, and keep the pond before and after changing the water. The salinity is basically stable, and the salinity of aquaculture water is maintained at about 20, and the outdoor earth pool should be changed once in a rainstorm and thunderstorm to prevent the pool water from being too light and the salinity in the pond to be abrupt, resulting in unnecessary losses.

2 Removal of debris and residual baits Every morning, organic debris and dead seedlings such as bait should be removed to avoid damaging the water quality, so that the crab seedlings will develop and grow under good water conditions.

(3) Disease control Although the crab has strong resistance to disease in the natural sea area, it will also cause many diseases under artificial breeding conditions, especially high-density, intensive, and large-scale farming. At present, there are few researches on disease prevention and control of young crabs during intermediate breeding. However, the prevention and control of blue crabs should adhere to the principle of "prevention first, prevention and control, and prevention and prevention of treatment." During the entire cultivation process, pathogenic factors should be excluded as much as possible to enhance the body and disease resistance of the blue crab in order to effectively control the occurrence of various diseases during the cultivation, increase the survival rate of young crabs, and reduce the losses caused by diseases. The main technical measures for preventing and treating diseases of big eye larvae and young crabs include:

A. Clear the disinfection work of clear ponds and disinfect the crabs and tools. During the entire cultivation period, maintain good water quality. Replace clean water frequently. When entering the sea water, strictly filter and remove the baits in the pool. Spill disinfection of drugs, such as oxytetracycline 2ppm or tea dry extract 2ppm or quicklime 10-15ppm and so on.

B. Feed fresh and high-quality bait, strengthen the nutrition of biological bait, artificially add appropriate additives such as polysaccharide, vitamin C, E and other immune enhancers to enhance the juvenile crab's constitution and its own immunity, thereby increasing its resistance to disease.

(4) Daily observation Each morning, evening and mid-night, the observation ponds should be observed once, observe changes in water quality, check the activities of young crabs and their feeding conditions, and pay attention to the occurrence of predators and diseases, and the setting of concealed objects in ponds. Also check the condition of the inlets and outlets and other facilities, find problems and take timely effective measures to deal with them.

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