Detailed Explanation of Key Points for Large-Specification River Crab Culture Techniques

Crabs are deeply favored by consumers at home and abroad for their rich nutrition and delicious meat. At present, the river crab has become a major aquaculture species in various regions, and the river crab farming industry has shifted from the “mass crab farming” that pursues yield to farming large crabs and “ecological crabs” centered on quality and efficiency. The ecological culture techniques for cultivating large-specification river crabs are mainly:

Select superior breeds and ensure strict quality control of stocked crab seed

Large-specification river crabs must be based on high-quality crab seed. Select self-bred or locally bred juvenile Chinese mitten crabs (Eriocheir sinensis) from the Yangtze River system that are robust, disease-free and injury-free, uniform in size, with intact appendages, glossy, and strong in activity as crab seed. Avoid using juvenile crabs of unknown origin. This is the most important factor in farming large crabs and improving the survival rate. Generally, high-quality crab seed can be selected from 3 aspects: identify crab seed from the Yangtze River system by several physiological characteristics such as appearance, body shape, walking legs, and frontal teeth; remove the “small dull crabs” that have no feeding value due to premature sexual maturity. The so-called “small dull crabs” refer to one-year-old juvenile crabs of the current year that reach sexual maturity at a body weight of 10-35g, stop growing, and die one after another; select healthy and high-quality crab seed based on the degree of walking leg damage, seed activity, and whether there are parasites on the body surface.

Plant aquatic plants to create a good ecological environment

As the saying goes, “The size of crabs depends on aquatic plants.” Planting aquatic plants in adult crab culture waters serves the following purposes: first, as natural high-quality plant-based feed for crabs; second, to provide hiding places for river crabs to inhabit and molt, making it harder for predators to find them and reducing mutual killing; third, the photosynthesis of aquatic plants can increase dissolved oxygen in the water and absorb organic matter in the water body, preventing water eutrophication, purifying water quality, maintaining fresh water quality, and improving the aquaculture environment of the water body; fourth, in high-temperature seasons, aquatic plants can provide shade and lower temperature, which is beneficial to river crab growth. The species of aquatic plants planted should not be single but diversified. It is best to combine submerged plants, emergent plants, and floating plants with reasonable distribution to suit the various needs of river crabs. For submerged plants, one can choose Vallisneria, Hydrilla verticillata, Elodea, etc.; for emergent plants, such as reeds, wild rice stem grass, etc.; for floating plants, such as duckweed, etc. The area planted with aquatic plants in crab culture waters should account for 50-60% of the water surface area. Summer July-August is the peak period for river crabs to feed on aquatic plants, so close attention should be paid to ensure both utilization by river crabs and a relatively high reserve. If insufficient, measures should be taken to supplement aquatic plants, but never introduce too many aquatic plants. Decaying aquatic plants easily cause water quality deterioration and induce river crab diseases. Too many aquatic plants, especially too much of a single species such as Elodea, can easily cause the bottom water body to be stagnant and deteriorate. Water body channels should be dredged in time to facilitate water inflow and drainage and improve crab pond water quality; otherwise, diseases such as black gills and edema are prone to occur.

Control stocking density and pay attention to reasonable polyculture

The stocking density of crab seed is related to crab seed specifications, rearing conditions, required market specifications, management level, etc. A two-year culture model can be adopted: purchase crab seedlings at the end of May and early June, carry out extensive nursery rearing in the pond, and by the end of the year they can be bred into button crabs; in the second spring, put these button crabs into the pond. Usually, the stocking specification of crab seed is 100-200 crabs/kg. Stock 300-500 crabs per mu of water surface. Appropriate release can improve the pond-out specifications and quality of river crabs. In adult crab ponds mainly for river crabs, herbivorous fish (grass carp, blunt snout bream, etc.), omnivorous fish (common carp, crucian carp, tilapia, etc.), and carnivorous fish (snakehead, catfish, bass, etc.) cannot be polycultured; only some filter-feeding fish (silver carp, bighead carp, etc.) can be polycultured. The purpose is to remove plankton in the water and prevent water quality deterioration. However, because crab ponds have many aquatic plants and the pond water is relatively clear and lean, the polyculture quantity should not be too large. Generally, polyculture per mu of water surface is 40-50 fish of 50-100g/fish silver carp and bighead carp seed (silver carp to bighead carp ratio of 5:1). In addition, soft-shelled turtles cannot be polycultured in crab ponds. If conditions permit, a culture model of polyculturing an appropriate amount of freshwater shrimp in crab ponds can be adopted, using ecological culture management measures to obtain high yield, high quality, and high efficiency.

Stock snails to meet the river crab's need for biologically active substances

Snails are the fresh live animal bait most favored by river crabs. Snails are benthic, slow-moving, and strong in water purification. In adult crab culture ponds, stock 150-250kg of live snails per mu of water surface and let them reproduce naturally. In this way, a continuous supply of palatable, active protein bait can be provided throughout the entire growth process of river crabs. Among various snail species, one can choose the excellent variety with the largest cultured individual among freshwater snails —— the apple snail. The apple snail is also called the golden apple snail. Its shell is thin, the individual is large, and the meat content is high (edible components account for 38% of body weight). It has omnivorous feeding habits, fast growth, a long reproduction period, and a high multiplication multiple. Transplanting apple snails into crab ponds can not only maintain fresh water quality in crab ponds but also provide river crabs with natural bait rich in various biologically active substances, promote rapid growth of river crabs, and improve the market specifications of adult crabs.

Regulate water quality to promote healthy growth of river crabs

River crabs have relatively high requirements for water quality: dissolved oxygen in water should be maintained at 5mg/L, pH 7-8.5, transparency about 30-40cm. The water level throughout the year should be shallow in spring and deep in summer, autumn, and winter, maintaining a good water environment in the crab pond with good, living, tender, and refreshing water quality to promote healthy growth of river crabs. During the culture process, regular water injection must be adhered to. In the early stage, change water once every 10-15d, each time changing 1/3. In high-temperature seasons, to ensure fresh water quality, add some new water every day and change water 2 times every 1 week. In addition, every 15-20d, use 8-10g of quicklime per cubic meter dissolved in water and splash it over the whole pond. If conditions permit, it is best to regularly use microbial preparations such as photosynthetic bacteria or EM bacteria dissolved in water and splash them over the whole pond to improve water quality.

Do a good job in crab disease prevention and control

In recent years, with the continuous expansion of crab farming scale, crab diseases have also continued to spread and escalate. Crab diseases that never existed before now also occur in large numbers, mainly ciliate disease in juvenile crabs and shivering disease in adult crabs, bringing huge economic losses to the crab farming industry. Therefore, crab disease prevention and control should adhere to prevention first, achieve early discovery and early treatment, prevent disease before it occurs, and treat disease early when it occurs, adopting different methods for prevention at different growth stages. Use microbial preparations such as photosynthetic bacteria and EM bacteria to regulate water quality. In high-temperature seasons, regularly use quicklime, dibromohydantoin, etc. for prevention. During the high-incidence period of ciliates, use crustacean-cleaning or ciliate-cleaning insecticides once every 15-20d. To avoid drug resistance, drugs should be used alternately. Shivering disease is a microorganism between bacteria and viruses. Using decamethrin to clear the pond is an effective method for preventing and controlling shivering disease.