Detailed Explanation of New Techniques for Artificial Freshwater Farming of River Crab

River crab, commonly known as crab, also called hairy crab, mitten crab, etc. It belongs to the class Crustacea, family Grapsidae. The cephalothorax is squarish-oval, generally 6-7 cm long, usually green. The chelipeds are powerful and densely covered with downy hair, and the walking legs are flattened laterally and long. Wild river crabs live in the muddy beaches of the embankments of rivers, lakes and marshes, and are produced along the coastal areas of northern and southern China. Because of its unique habit of growing in freshwater and migrating to the sea to reproduce, it provides natural conditions for artificial farming. Its artificial farming methods are briefly introduced as follows:

1 Pond construction. Choose a place with good water quality, sufficient water source, convenient drainage and irrigation, and ample sunlight to build the river crab farming pond. The excavated terrain should be neat, with an east-west oriented rectangle being best. The pond depth is 1.5 m and the water depth is 0.8 m. Because river crabs have a very strong climbing ability. They will also burrow and escape. Therefore, a wall should be started from 20-30 cm below the pond bottom, with a height of 150-180 cm. The top should use bricks or cement boards as an eave, in a shape, with an eave width of 15 cm, to prevent river crabs from climbing and escaping. The wall surface should be smoothed with cement, and the inlet and outlet should be guarded with aluminum wire mesh to strictly prevent escape. At the pond bottom, earthen long ridge-like mounds should also be made, and some aquatic plants such as wild rice stem and alligator weed should be planted along the ridge ditches, to provide places for river crabs to burrow and rest. The aquatic plant coverage should reach about 70%. If grass cannot be planted, some aquatic plants can be collected and placed at the edge of the pond water. This can also play the same role as planting aquatic plants.

2 Seedling transport. According to the characteristic that river crabs spawn and reproduce in the sea and then grow and develop in freshwater, artificial farming must transport seedlings over long distances. At present, although there have been major breakthroughs in artificial breeding of crab seedlings, it is still limited to key research institutions and cannot meet the demand for extensive rearing in various places. The Yangtze River estuary in China is a natural breeding ground for crab seedlings, and more than twenty provinces and cities across the country buy crab seedlings there, transport them back and release them for rearing. But crab seedlings must be transported properly to increase the survival rate. After repeated research and experiments, the Jiangsu Provincial Aquatic Technology Extension Station finally successfully developed the method of transporting crab seedlings in double-layer plastic bags with oxygenated water. That is, put 3-5 kg of water into a double-layer plastic bag, control the salinity at about 4%, then put 100-150 g of crab seedlings into each bag, then fill with oxygen, with an oxygen volume of 10-12 L. Finally, tie the mouths of the inner and outer plastic bags tightly, put them into a cardboard box and carry them away. One person can carry 10 bags at a time, that is, 1500 g, transport 400 km, travel 8-13 days, and the survival rate can still reach more than 80%, with the highest reaching 95%. It is an economical, effective, simple and easy method of transporting crab seedlings.

3 Stocking. 40 days before purchasing crab seedlings. The rearing pond should be disinfected with quicklime plus bleaching powder. Generally, for every 0.1 mu, use 10 kg of quicklime and 0.5 kg of bleaching powder, spread them into the pond water (generally leaving 3-6 cm deep), and stir repeatedly to thoroughly eliminate harmful organisms and aquatic enemies in the water. Then drain all the water, rinse once with clean water, and continue disinfection by exposure to sunlight. 15-20 days before releasing the seedlings, clean water can be filled in. Generally, one can go out to buy seedlings in May-June. Newly transported crab seedlings cannot be put into the pond immediately, but the plastic bags should be placed in the rearing pond or temporary holding pond for half an hour. When the inside and outside temperatures are close, open the plastic bags and pour the crab seedlings into the pond. Usually every 0.1 mu can stock 50,000-100,000 crabs, requiring 300-600 g. If water quality and feed conditions are good, the density can be appropriately higher. When releasing crab seedlings, the pond water should be shallower, about 0.5 m deep, to raise the water temperature. Reduce death and promote growth. After one week of rearing, increase the water level day by day until it reaches a depth of 0.8 m. For juvenile crabs reared for one year, 800 can be stocked per 0.1 mu, released in April-May.

4 Feeding. River crabs have omnivorous feeding habits and a wide range of feed, mainly animal and plant feeds. Animal feed mainly includes small trash fish ground into paste, river snails, fish offal, livestock and poultry blood, earthworms, fish meal, etc.; plant feed mainly includes soybean powder, corn flour, wheat flour, bran, rice bran, etc. Generally, half animal feed and half plant feed is best, and a small amount of green feed such as aquatic plants should be added, mixed evenly and then fed. For the feed of newly released crab seedlings, feeding fish and shrimp paste is mainly used. In daily feeding, adhere to the principles of "four observations" (observe the season, observe the weather, observe the water color, observe the feeding and activity status of river crabs) and "four fixings" (fixed location, fixed amount, fixed time, fixed quality). The time is best at 8-10 am and 3-5 pm. The feeding amount is generally based on the feed put in each time being eaten up within two hours. The feeding amount should gradually increase with the growth of river crabs. Generally, for every 10 kg of river crabs, feed 0.5 kg per day. The aquatic plants in the pond can serve as feed and also provide shade and prevent heat, and are indispensable for raising river crabs, so great attention must be paid to them.

5 Management. River crabs like to live in places with fresh water quality and aquatic plants, and are especially sensitive to lack of oxygen. Therefore, the rearing pond must be regularly supplied with new water. In the early stage, change the water once every 10-15 days, and in the high-temperature seasons of summer and autumn, change the water once every 2-3 days. Each water change should be 2/3 of the pond water. Smelly pond mud should be removed in stages. If the pond water is too fertile, more aquatic plants such as alligator weed can be planted along the pond edge. In this way, river crabs can hide in the water to molt, and can avoid killing each other. At the same time, aquatic plants can also absorb nutrient salts in the pond, increase dissolved oxygen in the water, prevent water quality deterioration, and benefit the growth of river crabs. River crabs also have many natural enemies, so prevention and control work should be strengthened. Their main enemies include rats, water snakes, water birds, frogs and water centipedes, etc. According to reports, one rat can eat or bite to death 10 river crabs a day, and is the most vicious enemy. Various effective methods should be used to kill and prevent rats. When water snakes, water birds, frogs and water centipedes are found causing harm, they should be promptly beaten and driven away to ensure the normal growth of river crabs.