Eco-friendly farming technology for Chinese mitten crabs in river ditch enclosures
River ditch selection
Crab-farming river ditches require fresh water quality, no pollution upstream, a relatively flat bottom, and not overly thick silt, with a transparency of 30-40cm, dissolved oxygen not lower than 5mg/L, a year-round water depth of 1.0-2.5m, small water level fluctuation, and a certain gentle flow. The river ditch should have lush aquatic plants, mainly hydrilla (Hydrilla verticillata), eelgrass (Vallisneria), and Malaysian pondweed (Potamogeton malaianus), with a small amount of hornwort and alligator weed. Convenient transportation is needed to facilitate management and transport.
Design and installation of the blocking net
Crab farming in river ditches should establish safe and reliable escape-prevention facilities, and the strength of the equipment must match the highest water level and maximum flow rate of the river channel. The structure of the blocking net consists of a wall net, supports, and stone cages. The wall net is the main part, generally woven from 0.21/3×4 polyethylene thread, with a mesh size of about 1.5cm, and double main ropes of 5mm diameter fitted at both top and bottom. The wall net is 4.0-4.5m high, with double layers: the inner net prevents escape, and the outer net blocks debris and prevents intrusion by wild miscellaneous fish, serving a protective role. The inner and outer nets are spaced 2-3m apart, with a bag net installed between them, which can be used to check whether crab seedlings have escaped or wild miscellaneous fish have entered, so as to determine whether the blocking net is damaged. The stone cage is a facility for fixing the bottom main rope of the wall net. It is made by sewing mesh sheets (mesh size 2-3cm) woven from 0.21/3×4 polyethylene into a cylindrical net bag 15cm in diameter, filled with stones 3-4cm in diameter. When setting the net, it is sewn along the bottom main rope to the wall net and pressed 35cm into the bottom mud. At the top of the wall net, a 40cm-wide plastic film is sewn on the inner side of the mesh to prevent river crabs from climbing out and escaping. Every 2m, a bamboo or wooden stake 12cm in diameter and 4-5m high is driven in, inserted 0.5-1.0m into the pond bottom, and connected into a whole with crossbars, with the wall net tied to the stakes to fix the net shape.
Preparation before stocking
1. Clearing wild species and pests: Generally, river ditches contain some wild predatory fish such as snakehead, catfish, and carp, which must be thoroughly removed; otherwise they will affect the habitat and feeding of river crabs. Especially shortly after molting, river crabs are soft-bodied and have poor mobility, and are easily preyed upon by these pests, reducing the recapture rate. Electric fishing, net fishing, and other methods can be used, starting from one end and clearing section by section, advancing gradually; chemical treatment can also be used starting from the upstream end.
2. Selection of crab seed: Natural or artificially bred button crabs from the Yangtze River system are preferred. If conditions permit, self-bred button crabs can be used to reduce costs. They should be uniform in size, robust in constitution, with complete appendages, active in crawling, free of disease, injury, and attachments, and sexually mature old crabs should be removed. Crab seed purchased in the same batch should be placed in the same enclosure area.
3. Setting up a temporary rearing area: Before stocking, use mesh sheets with a mesh size of 1.2cm to enclose a temporary rearing area, accounting for 10%-20% of the entire enclosure area, and strengthen feeding management. When the aquatic plants sprout and grow relatively lush in April-May, remove the mesh sheets so that the crab seed can crawl into the enclosure area on their own to forage, allowing the aquatic plants to maintain strong regenerative capacity.
4. Treatment before stocking: For crab seed purchased from other places, soak them together with the packaging bags in water for 2-3min, rinse off the foam, lift them out for a moment, then soak them in water again for 2-3min, repeating 3 times. After the crab seed have absorbed enough water, open the packaging, take out the crab seed, and disinfect them with drugs, generally soaking in 3%-5% salt water for 3-5min or in 10-20g/cubic meter potassium permanganate for 10min, then place them in the temporary rearing area.
Stocking time and method
The stocking time for river crabs is late December each year or before the end of March the following year, with a water temperature of 5-10℃ being preferable, and it is best to stock enough in one go. The stocking size is 150-200 crabs/kg, with 7500-9750 crabs per square hectare; large-size commercial crabs can be obtained by the end of the year of stocking. To make full use of the water body and food organisms, silver carp at 4-6 fish/kg can be stocked at 75-150 fish per square hectare, bighead carp at 150-220 fish per square hectare, and culter at about 15cm at 220-300 fish per square hectare.
Maintenance and management of the aquatic ecological environment
1. Cultivating and transplanting high-quality aquatic plants: Aquatic plants are not only good food for river crabs, but also places for them to inhabit, hide, and molt, and they can purify water quality; in summer, they can also prevent heat and lower temperature, promoting the growth of river crabs. River ditches themselves grow some high-quality wild aquatic plants, and the growth and reproduction of these aquatic plants should be ensured, while planned transplanting and supplementation are also needed. For example, in early winter, collect eelgrass seeds, soak them in water for 10-15min, mix them evenly with mud, and broadcast them on the river bottom; each kilogram of grass seed can be sown over 0.3-0.5 square hectares. Alternatively, gather hydrilla from other water bodies, cut it into small sections of about 15cm, and insert them into the water bottom; transplant Malaysian pondweed in February-March at 750-1500kg per square hectare, divided into small clusters and evenly inserted with bamboo poles 5cm deep into the river bottom. During the farming process, submerged plants should be kept 20-30cm below the water surface, and the excess should be cut off, controlling the growth of alligator weed and hornwort so that the aquatic plant coverage reaches about 80%.
2. Transplanting snails: Snails can reproduce on their own at the bottom and provide high-quality live food for the growth of river crabs; at the same time, they can feed on unicellular algae and organic debris at the bottom, improving water quality. Around Qingming each year, live snails can be released in the enclosure area at 3000-4500kg per square hectare.
3. Reasonable feeding: Throughout the farming process, according to the different seasons, follow the principle of “fine feed before and after, green feed in the middle, and a mix of meat and vegetables.” From late March to May, when the water temperature is relatively low, feed fresh small miscellaneous fish cooked and mixed with a small amount of wheat flour into dough-like lumps, placed at multiple points along the riverbank as the first feed, once a day at 2 p.m., with a feeding amount of 2%-3% of crab body weight, or choose complete pellet feed with an appropriately reduced amount. In June, when the water temperature rises, feed twice a day, with 30% of the daily feed amount at 8-9 a.m. and 70% at 5-6 p.m.; at this time, increase the feeding amount to 5%-8%, mainly using small miscellaneous fish and complete pellet feed, with soybean cake, wheat, corn, and others as supplements. From early July to early September is the high-temperature season, with feeding twice a day, mainly plant-based feed such as corn, wheat, broad beans, soybean cake, pumpkin, potato, and aquatic plants, supplemented with pellet feed, with a daily feeding amount of 3%-5%, and fresh feed 8%-10%. In mid-September, river crabs enter the fattening period, and animal feed should account for 60%, supplemented with 40% plant-based feed, with a daily feeding amount of 8%-10%, then decreasing to 3%-5% after mid-October.
4. Daily management: In the early stage after crab seed stocking, they are relatively active and prone to escape, so it is necessary to patrol the river and nets every day to check whether any crabs have escaped. In the middle stage, do a good job in wind prevention, flood prevention, and prevention of theft and damage; patrol frequently, repair frequently, and frequently remove floating objects such as aquatic plants, ensuring that the stakes are neat and firm, the blocking net is clean and free of holes, the stone cages do not come out of the bottom mud, and water flow inside and outside the net clothing is smooth. In the later stage, due to reproductive migration, sexually mature river crabs will desperately try to escape, so attention should be paid to strictly preventing river crabs from climbing over the upper part of the wall net and escaping, and catching should be carried out promptly, while organizing personnel to patrol the river and strictly prevent poaching.
5. Capture and temporary rearing of adult crabs: Because the blocking net facilities are limited to the water inlet and outlet, escape-prevention equipment cannot be built along the two banks of a narrow and long river ditch. This requires that river crab capture be carried out at the beginning of their natural reproductive migration, and that temporary rearing of adult crabs be done well. The capture time starts in late September, and various tools such as three-layer gill nets, crab cages, and fixed set nets can be used to catch them, or they can be caught by hand along the riverbank at night. Adult crabs should be weighed and counted, separated by size and sex, and placed in ponds for temporary rearing. Through artificial feeding for fattening, careful rearing and management, and timely marketing, achieve good crab farming, good crab sales, and increased product value.