The key points of the crab-rearing mode in net enclosure are as follows:

(1) Selection of net enclosure area

Crab-rearing in lake enclosure should meet the following conditions: First, the lake is open, the water quality is good, and the water flow is slow and unobstructed. Second, the water level is suitable, the annual water level is 1 ~ 1.5 meters, and the water level difference is small. Third, the bottom of the lake is flat, with clay, hard mud and little organic matter in silt. Fourth, the aquatic plants are dense, the natural bait resources are rich, and the enemy organisms are few. Fifth, it does not affect water storage, flood discharge and ship navigation, and the environment is quiet and the transportation is convenient.

(2) Fencing construction

Polyethylene mesh with mesh of 1-1.2 cm is used as fencing, and bamboo is used as pile. Pile and hang the net around the delineated culture area, with one pile every 1 ~ 1.5m. The bottom of the net is fixed with gabions and ground anchors, so that the foot of the net is close to the bottom mud and sinks into the bottom mud. The upper part of the net is 1 ~ 1.5m higher than the water surface, and the top is equipped with an inverted net with an inward inclination of .5 ~ .7m to prevent crabs from climbing out of the net. Generally, the net enclosure area is about 3 mu, and the maximum is not more than 1 mu. Usually, another net enclosure area is built outside the breeding area to ensure safety. There are also bamboo foil instead of mesh to form foil net. Each piece of bamboo foil is firmly inserted into the mud, which is not easy to be bitten by river crabs. The lake with thick mud has good lodging resistance, but its service life is short and its production is time-consuming. The net enclosure is convenient to make, but it has poor lodging resistance.

(3) Planting aquatic plants and protecting aquatic plants resources

The number of aquatic plants in lakes and net enclosures not only directly affects the quantity, specifications and quality of river crabs, but also relates to the key measures for sustainable development of crab culture in net enclosures. In order to protect the aquatic plants resources in the lake, on the one hand, we must protect the aquatic plants outside the purse seine and make rational development and utilization, on the other hand, we must plant aquatic plants inside the purse seine. The method is to divide the net circumference into two parts with net pieces. First, half of the water surface is used for stocking crab species, and the other half is used for planting aquatic plants. At present, Elodea nuttallii, Ceratophyllum demersum, Hydrilla verticillata and Sophora alopecuroides are mostly planted in production. The cultivation method of aquatic plants is the same as raising crabs in ponds. When the aquatic plants grow up, wrap one end of the aquatic plants into a ball with wet mud and throw it into the net of the other half of the non-stocked crab species. When it grows to about 5 cm, remove the separating net to let the river crabs enter the aquatic plants area.

(4) Crab breeding

To raise adult crabs in the Yangtze River basin, it is necessary to buy crabs from the Yangtze River system to avoid "Liao crabs" and "Ou crabs". Avoid buying mature crab species, and always pay attention to the quality requirements of crab species, such as no disease or injury, strong physique, neat specifications, thin shell, thin navel and close to the body. After buying the crab species, soak them in 1/1 fresh water or 3 g/L salt water for 1 ~ 15 minutes to prevent the occurrence of diseases and improve the survival rate. If possible, it is best to cultivate the crab species by local crab seedlings themselves, and the crab species should be cultivated at the water temperature of March. Generally, 2 ~ 4 species of crabs are raised per mu of water surface. Generally, mixed culture of fish and crabs is adopted in cage culture, and fish species are still raised as usual. However, some herbivorous fish should be reduced in stocking structure, and some crucian carp, silver carp and bighead carp should be added to alleviate the competition between fish and crabs.

(5) Strengthen feeding management

① Increase feeding: Crabs in purse seine can't meet the growth and development needs of river crabs only by relying on natural feed, so we should pay attention to the supplement of feed, properly protect aquatic plants and snails in the breeding area, maintain ecological balance and avoid predatory utilization. Small fish and shrimp, snail meat, animal scraps, cereals and cakes should be fresh, and corruption and deterioration are strictly prohibited. According to the water temperature, the amount of natural bait in the weather and the feeding situation of river crabs, the feeding amount should be flexibly controlled. Generally, the daily feeding amount is 3% ~ 5% of the total weight of river crabs. More feeding should be done to ensure that everyone can eat feed.

② Transplanting aquatic plants: Aquatic plants are not only high-quality bait for river crabs, but also provide a hidden environment for them to shed their shells and improve water quality. Therefore, aquatic plants are essential for the cultivation of river crabs. If there are few aquatic plants in the cultivation area, attention should be paid to transplanting them, and the aquatic plants can occupy 2% ~ 3% of the water area.

③ Pay attention to water quality management: The water quality management of crab culture in purse seine is mainly to remove dirt, rotten aquatic plants, etc., and it is forbidden to discharge sewage and soak hemp in water. At ordinary times, it is necessary to clean the purse seine to prevent mesh blockage from affecting water exchange. In addition, because purse seine culture is carried out in shallow water, sometimes there will be a phenomenon that aquatic plants are too strong and dense, which will affect water exchange. At this time, attention should be paid to removing too many aquatic plants. If it is difficult to remove them, a channel with a width of about 2 meters can be opened every 2 ~ 3 meters to ensure smooth water exchange.

④ Strengthen disease prevention: Crabs are prone to get sick in the hot summer season. Sprinkle 15-2g/m3 of quicklime on areas where crabs are concentrated every 15-2 days to prevent diseases, pay attention to observation at ordinary times, and take effective measures to treat diseases in time.

⑤ Prevent escape: The purse seine must be firmly set. Generally, double-layer purse seine should be used, and it should be firmly fixed with wooden stakes every 3-5 meters. Mud should be stepped into the bottom with a stone cage with a diameter of 15 cm, and the upper part should be covered with a net or plastic film to prevent the river crab from escaping. Pay attention to the inspection during the breeding season to see if the net is damaged, whether the wooden stakes are firm and whether the stone cage is displaced. If problems are found, pay special attention to them, especially after the storm and in the later stage of breeding.

(6) Catch crabs in

lake nets at the right time. Because of the superior environmental conditions, they grow faster than ponds and mature sexually earlier than ponds, their reproductive migration begins earlier. In the middle and lower reaches of the Yangtze River, they usually turn into green crabs in mid-September. Therefore, they usually start fishing in late September, and the fishing tools mainly include crab pins, artificial crab nests, ground cages and wire meshes.