Crab ecological breeding includes the selection and retention of pro-crab, wintering, production promotion, feeding of hatchlings (larva hatching), larval rearing, and crab fishing. Among them, pro-crab is the foundation of ecological breeding, and larval rearing is the key to nursery.
First, the selection of pro-crab crabs should be selected as the parent crabs cultivated in the intensive culture ponds of the Yangtze River water system, requiring a healthy body, complete appendages, body surface clean, no disease. The size of female crabs is about 80 grams. The size of male crabs is about 125 grams, and the ratio of male to female is 2-3:1. The selection and retention of the pro-crab crabs is generally carried out before and after the beginning of winter. However, according to the production conditions in Chongming for several years, farmers usually purchase the water at the temperature of 8 to 14°C in the middle and middle of March, and then directly put the pro-crabs into the production pool for mating, thus restoring the physical strength of the pro-crab and There is a certain influence on the adaptation of the water environment.
Second, the pro-crab mating crabs into the pond after a period of intensive cultivation, to be suitable for external conditions can be mated. According to the climatic characteristics of Chongming, the peak period of mating and spawning of crabs is in the middle and early March. During this period, the wintering crabs will be placed in 15‰-22‰ seawater and both will be able to successfully spawn. The second day after the sea water promotes production, the individual can see the egg. After one week, the rate of ovulation can reach 70%.
Third, compared with the pro-crab raising, the general management methods are basically similar to those of the seawater environment. The following four points should be followed:
(1) Male-female division must be maintained: All male crabs should be removed cleanly. If there are male and female crabs in mixed crabs, male crabs will chase the entangled crabs and repeat mating, which may easily lead to disability. Eggs were scattered and the number of male crabs was 2-3/m2.
(2) The amount of feeding should be increased appropriately: After the first time of spawning, the grazing crab needs a lot of food, accumulates nutrients, and prepares for the next ovum. Therefore, it is not only necessary to increase the amount of feed, but also to improve the variety of the food. Never let the crabs hungry, otherwise the crabs will use large claws to dig eggs and eat them, causing serious consequences for production.
(3) Maintain fresh water quality: Replace new water once every 3-4 days, and at the same time, pay attention to preventing seawater salinity fusion, especially when embryonic development is very sensitive to changes in salinity. The army is overwhelmed.
(4) About one month after the pro-crab is held, the development of the embryo must be carefully checked. When the embryonic heart rate reaches about 130 times per minute, the crab will be caught by the dry pond.
Fourth, larvae cultivation (1) clear pond disinfection work: In the larvae before the start of the cultivation of the first half to 1 month, it must be cleaned and sterilized cultivation ponds, kill the enemy of harmful organisms, clear the bottom of silt and maintenance into , drainage pipes, etc., to ensure the smooth development of the cultivation work. Before the hatchling crabs are put into a cultivation pond for hatching, they must be sterilized with drugs to kill bacteria and parasites that are attached to the crab body. When the water is injected, the precipitated seawater must be filtered with a gauze of more than 200 mesh, and the water level should be controlled at about 1 meter.
(2) Density of seedlings: Density of seedlings should be controlled to about 30,000-60,000 larvae per phase of water per cubic meter of water. This stocking standard, in terms of current breeding conditions and means, will help improve the survival rate and yield.
(3) Baits of juveniles: The juveniles of river crabs are omnivorous in various stages, mainly based on animal foods, supplemented by plant foods, and also feeding on organic detritus. Commonly used in production are rotifers, larvae, Artemia nauplii, soymilk and artificial particles with food. Feeding is an extremely important measure in the development of juveniles. The principle is "appropriateness, palatableness and appropriateness". It can or can do this "three adaptations" in order to enable the larvae to develop a higher survival rate and yield.
(4) Improve water quality: In the production of nursery, the problem of water quality is an urgent problem to be solved. At present, there is a general situation that the water quality is poor, which is also an important factor in the instability of nursery yield. Experiments have shown that the crab larvae during the molting metamorphosis, the physical and chemical changes in water quality and tolerant to the content of toxic substances decreased significantly, larvae usually occur in the molting or molting and just completed molting individuals. Therefore, special attention should be paid to changing the timing of the water, changing the amount of water and physical and chemical indicators of water quality, in order to create the most suitable water environment to meet the normal development of larvae, enhance physical fitness and improve disease resistance. According to previous years' production experience, placing aerators in the cultivation ponds yields higher yields than holding ponds. Because of the operation of the aerators, in addition to aeration, the contaminated water can form a flowing water environment suitable for larvae and prevent larvae. Assemblages play a role in the group and the bottom, so that the distribution of larvae and live bait is even, which is conducive to the promotion of larval growth and metamorphosis. In the production of nursery, there are several water quality indicators that the majority of farmers should understand. Oxygen content in water should be kept at 5 mg/l or more; suitable temperature is 19-25°C; optimal salinity is about 13 to 26‰; pH should be controlled between 7-9; non-ionic Ammonia nitrogen should be controlled below 1.5 mg/l.
(5) Disease prevention measures: The integrated prevention and treatment measures for disease during nursery stage should adopt integrated prevention and treatment measures that are prevention-oriented and supplemented by governance, and should be taken seriously in all aspects. Including the choice of non-bacterial pro-crab, pro-crab after purchase, disinfection, regular disinfection of pregnant crabs, in the larvae will soon be stripped into the hatching before disinfection and so on. In view of the increase in diseases of crab larvae, a large part of the disease is due to the sickness of the embryos of the hatching crabs, so the disease should be eliminated before hatching, and do not bring the disease into the larval rearing tank.
Biological disease prevention is the only way for future disease prevention and control. Put a certain amount of photosynthetic bacteria in the nursery pond to form a dominant community in the nursery pond, decompose organic waste, reduce the concentration of harmful substances such as ammonia nitrogen and hydrogen sulfide in the water, and keep the water quality fresh. The disease-resistant enzymes that are released when photosynthetic bacteria reproduce in water can also prevent the disease of crab seedlings to a certain extent. In the early stage, it can also be used as juvenile bait, which contains rich B vitamins and other active substances, can promote the growth of crab seedlings, enhance the body resistance to disease.
At the stage of susceptibility to larvae, it is effective to use some bactericidal drugs appropriately, but it is necessary to prevent the side effects caused by insufficient doses and improper methods for long-term use of drugs or drugs. Usually 3-5 days spilled 0.1-0.2ppm potassium permanganate once, or with oxytetracycline feeding bait, commonly used amount of 1% of bait. In order to improve the bottom of the nursery pond, lime, zeolite powder and the like can be added.
(6) Good food and reasonable feeding: The bait is the material basis for the larvae to maintain metabolism and growth. Therefore, the palatability, nutritional value, and freshness of the food should be ensured, and the food should not be corrupted or disinfected. In addition, the type of bait and feeding method should be arranged according to the different stages of the larval development. In principle, live bait is mainly used, supplemented with substitute bait if necessary, and feeding should follow a small number of principles.
Fifth, crab fishing and desalination When the pool of about 80% of the sickling larvae metamorphosis to bigeye larvae, can begin fishing. The advantage of this is that it can reduce the loss of fifth-stage zoea that have not metamorphosed into crabs in the pool by large-eyed larvae, and improve the survival rate and yield of larvae. However, crabs should not be caught too late. If crabs have already become young crabs, it is not easy to catch them.
According to the clustering and phototaxis of the crab seedlings, the seedlings can be used during the day to use the crab seedlings, like a certain flow, hi the shore habits, choose the upper corner, through the rotation of the aerator, causing a certain direction of the flow, and then use the nets Copying and catching can catch large amounts of crab seedlings. In the evening, light trapping method can be used, mainly utilizing the light habit of the crab seedlings, using 100-200 watts of light to trap and copying with a net.
Crabs removed from the cultivation ponds must be dewatered for three to four days before they are ready for stocking in freshwater waters before they can be stocked in freshwater waters.
The method of raising large-eyed larvae is to set a gas stone per square meter in the pool, continuously supply a large amount of gas, hold a density of 50,000 to 100,000 per cubic meter, and feed and exchange water several times a day. About 1/3. After being raised, the large-eyed larvae have a strong and lively body and a sturdy appearance, and their specifications can reach 140,000-160,000 per kilogram. The visual standard is to grab a large larvae that have been drained by hand and gently pinch. When the palms are released, the big eye larvae quickly spread out to escape. Such large-eyed larvae can withstand long-distance transport and have a high survival rate.
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