Efficient artificial culture technology of green crab

Scylla serrata referred to as green crab, commonly known as sturgeon. Guangdong is called crab, Taiwan and Fujian are called red sturgeon; southern Zhejiang is called crab, Philippines is called mud crab. Because its head breastplate was blue, the front edge with 9 side edge teeth was serrated, so the name saw edge crab.

Green crab is a famous edible crab with high nutritional and commercial value. It was cultured in Humen, Guangdong Province as early as 1891. It has a history of more than 100 years and is famous at home and abroad.

Scylla fry stocking

Scylla breeding seed sources, one is artificial breeding; two is fishing sea area megalopa (crab); three is to capture natural crab species. The artificial seedlings are neat and pure, and are still in the experimental stage at present. The large-scale productive seedling technology has not yet been comprehensively broken through. Before the breakthrough of artificial breeding technology, catching crab larvae and crab seeds in natural sea areas is still the main source of crab larvae in artificial culture.

1. Catch megalopa

① Crab fry fishing: green crabs lay eggs in the sea area with high salinity. After hatching, the larvae metamorphosed into megalopa and gradually moved to the estuary and inner bay to inhabit in the slightly lower salinity environment. At this time, it was the fry season. Natural crab seedlings can be caught from April to November along the coast of Zhejiang Province, and their peak seasons are May to June and August to September. Crab fry fishing methods are mostly set net, push net and hand-copied net three kinds. All localities shall select them according to the specific conditions of tidal current and wind waves.

② Identification of crab larvae: In the caught natural crab larvae, there are often mixed megalopa of other crabs, which need to be identified and selected. See Table 3-4 for identification classification.

③ Juvenile crab cultivation: Juvenile crab cultivation refers to the process of strengthening the cultivation of crab seedlings caught in natural sea areas or artificially cultivated into juvenile crabs. After cultivation of juvenile crabs, individual growth, increased resistance, culture survival rate is higher.

2. Catch natural juvenile crabs. Natural juvenile crabs, individual size is different, it is required to catch juvenile crabs with strong physique, complete appendages, no mechanical damage and few attachments. Zhejiang coastal juvenile crabs concentrate on seedling development period from June to mid-July (also known as "summer crabs" and "plum crabs") and from mid-September to mid-October (called autumn crabs). Summer crabs can directly develop commodity specifications in the same year, and autumn crabs can develop commodity specifications only after overwintering. The open air time of green crabs after catching should be short, especially in summer when it is hot and humid. Generally, when the temperature is above 28°C, it should not exceed half a day, and when it is below 25°C, it should not exceed 2 days. The shorter the time from capture to stocking, the better. Too long can cause death.

3. Quality identification and selection of green crab seedlings

The crab seedlings caught by themselves in the local sea area have a high survival rate due to the basically same environmental conditions and short exposure time. For example, the crabs that have been transported for a long time must be strictly selected and can be bred only after the sick crabs are eliminated. The methods for seedling selection and identification are as follows:

① Select healthy breeding crabs. Healthy green crab seedling shell is green, complete, complete body without damage, sensitive sensory response, strong mobility, not easy to catch; swimming feet and cheliceps can not be missing and disabled, step foot lack can not exceed 3, if step foot broken or injured, the remaining foot can be broken at the joint between the base segment and the seat segment, new appendages will be regenerated in a short time, if not broken, the stump will flow a lot of body fluids and cause death. Where the shell, abdomen and appendages have different colors (such as dark blue, reddish brown, rust, etc.) by thorns, hooks, sunburn crab seedlings of poor quality, not to choose as well.

2 No disease. Disease crab can be identified from the foot base muscle color to judge, its flesh color was blue, limb joint muscle does not sink, elastic for healthy seedlings, such as muscle was yellow-red or white spots, limb joint muscle sink, no elasticity, it is not suitable for breeding.

(3) Remove crab slaves, there are a few crab larvae parasitized at the inner base of abdominal segment 1 or more crab slaves, crab slaves are oval, soft body to absorb nutrients in the body to maintain life, parasitized green crabs affect normal growth and development, so crab slaves should be removed in time when selecting.

④ Crab larvae transportation vehicles are generally hard basket frame or wooden box. Spread a layer of wet grass at the bottom, put a layer of crabs, and then cover a layer of wet grass, so that young crabs will not be bruised. Do not overlap too much, and finally cover the hard frame screen cloth to facilitate the way to sprinkle seawater and improve the transportation survival rate.

seedling stocking

1, stocking specifications and time: summer seedling stocking is generally in June to July, specifications of 15~50 grams (average 25 grams), after 3~4 months of breeding, the year can reach the commercial crab specifications; autumn seedling stocking is generally in September to October, specifications in 10~20 grams, overwintering culture to May to June of the next year, can reach the commercial specifications. If the overwintering stocking quantity is insufficient, 50~100 grams of crab seeds can be replenished in March to April of the second year. Fattening breeding, generally 9~10 months to receive more than 150~200 grams and have mated thin crabs, breeding 30~40 days can reach the crab.

2. stocking density. From April to May, 50~ 100g/overwintering crab is bred, and 200~500 seedlings per mu are appropriate. If the specification is small, the seedling density should be increased appropriately. It can be caught in rotation about two months. At this time, it is the season for summer seedlings to grow, so small-sized summer seedlings can be bred, and the number of seedlings per mu is about 1000~1500 per mu. Fattening breeding time in August to October, stocking density depending on environmental conditions can be controlled at about 500/mu is appropriate.

cultivation management

1. Feeding bait

Baiting is one of the important methods to obtain the yield of green crab. According to the life habits of green crab, reasonable feeding technology can effectively play the superiority of beach purse seine crab culture. Green crab food to make full use of natural bait in the pool of reproduction or through the mesh into the pool of bait organisms. Green crabs are mainly carnivorous, like to eat shellfish and small crustaceans, can feed live blue clams, short-toothed clams, duckbill clams and other low-value shellfish, some beaches are originally some low-value shellfish growth and breeding areas, should be protected and utilized, but also can be moved into some annelids (such as sand worms) and small benthic crustaceans, self-reproduction ability is strong, green crabs like to eat, these are good live bait for green crabs. In addition, small miscellaneous fish and shrimp and freshwater shellfish (such as snails), artificial food, etc. can be used as a supplement when there is a lack of bait.

According to the foraging habit of green crabs, artificial feeding is best carried out in the evening. Feeding amount depends on the season, tide, water quality and natural bait in the pool to master, generally speaking, the sea area water temperature is appropriate (18~25°C) and during the flood season can increase the feeding amount, can be 5%~10% of the total weight of crabs in the pool feeding, in the flood season and rainy season when the climate is abnormal should be a small amount of feeding. Baiting places should be evenly sown and withdrawn on the beach. After feeding, check the feeding situation of green crabs. If there is any bait left on the next day, stop feeding and find out the reason.

2. Water quality management

Seawater can flow in and out naturally in flood season of high tide, water body in the pool can be fully exchanged, and there is no water quality problem. On the eve of arrival of low tide, check and repair leakage water of earth dam, raise overflow outlet elevation, store full water level, generally keep water level at about 0.5m on beach surface, in case of heavy rain, let overflow dam overflow upper fresh water, so as to prevent sudden drop of seawater salinity from causing green crab disease.

daily inspection

The key to success or failure of crab culture in beach purse seine is the ability to resist wind and waves of facilities and the measures and effects of preventing escape. Therefore, the breeder must patrol and watch every day. After each low tide, check whether the earth dam has seepage or leakage, whether the purse net pile collapses, whether the purse net is damaged, and repair it in time if any problems are found. The green crab is active frequently at night, and it is easy to stimulate the escape of the net at high tide. When the tide overflows the dam, the green crab turns around the purse net. If there is a hole in the purse net, groups of green crabs escape to this place. At this time, repair the broken net clothes immediately. In addition, when the water quality in the small flood pool is poor, it is easy to make the green crab dig a hole to escape, and attention should also be paid. The enemies of green crab cultured in beach purse net mainly include goby, Chinese snakehead, finger horse, etc., which are often eaten during the molting period of green crab. Therefore, during the breeding period, they can be caught in the dry pond after low tide in every flood season to eliminate the enemies, but they can keep mullet fish and shuttle fish in the pond, which is conducive to cleaning the sludge and residual bait at the bottom of the pond, and can coexist peacefully with green crab and increase extra yield.

harvest

Green crabs in intertidal enclosure should be collected in good time, and rotation measures should be taken to catch large and keep small, and catch fat and keep thin methods to obtain higher economic benefits. September is the mating season of green crabs, male crabs should be collected in due time, because male crabs after multiple mating, meat thin, reduce the edible value of meat crabs, and then a large number of deaths, female crabs after mating, about 30 to 40 days of breeding can become crab, if allowed to over-mature will lead to egg crabs, will also reduce the value. In addition, the water temperature in the sea area drops, and it is difficult for green crabs to overwinter in enclosure. It is also required to collect them in due time. In general, it is required to collect green crabs in the pond before the end of October. The collection methods of green crabs in enclosure are as follows:

1, light catch, according to the green crab at night like to climb the pool or tide habits, can be used light lighting, to copy the net fishing method.

2, dry pool hand catch and rake catch: freehand catch is an ancient and practical crab catching technology, crab body will not be damaged, but there must be skilled technology. Rake crabs with crab rakes. Crab can be basically collected net, more time-saving, but crab easily injured, operation should be careful when light rake. The green crabs caught should be tied up to prevent mutual bite injury. In summer, it should be stored in a cool and humid place when it is high temperature, and covered with water grass for heat preservation when it is cold.