What are the breeding methods for river crab seedlings in earthen ponds?

I. Earthen pond river crab seedling rearing Earthen pond river crab seedling rearing refers to the method of artificially rearing river crab larvae in excavated ponds using natural seawater in coastal areas. This method is low in cost, simple and practical, and is suitable for specialized crab farming households. In addition to the rearing pond, earthen pond crab seedling rearing also requires brood crab ponds, spawning promotion ponds, bait ponds, seawater sedimentation ponds and other facilities. Earthen pond river crab seedling rearing has a wide range of applications; as long as there is an area of unpolluted seawater with a salinity above 0.013, a pond can be built for seedling rearing. Its rearing conditions are closer to the natural ecology. Generally, earthen pond river crab seedling rearing takes place in the open air, so cold air intrusion must be strictly prevented to avoid a drop in water temperature; if conditions permit, warming facilities can be used. Alternatively, the seedling rearing time can be delayed somewhat; if the water temperature is relatively stable, the seedling survival rate can also be greatly increased.

II. Pond shape and structural requirements The pond shape should be square, with an area of 0.5-1 mu, a water depth of 1.5 m. Because the larvae like to cluster, like to face the wind and swim against the current, and like the edges and corners, most of the seedlings in the rearing pond gather at the upwind end of the pond. If the pond area is too large, the vast majority of seedlings concentrate together, which can easily cause excessive density of crab seedlings in a local area, leading to lack of bait, lack of oxygen and other problems, and thus massive death. The pond bottom should be sandy soil; the pond perimeter should be mortared with block stone (or cement precast slabs) into stone walls (or cement walls), so that when the water level changes, the larvae will not be stranded and die of drying. One end of the pond is equipped with a water inlet valve. The other end is equipped with a trumpet-shaped bottom outlet. The trumpet-shaped outlet should be blocked with GG54 nylon bolting cloth or polyethylene cloth to prevent larvae from escaping when the pond water is drained. III. Pond clearing and disinfection Pond clearing and disinfection are generally carried out 10-15 days before larval rearing, to kill harmful organisms (fish, shrimp, aquatic insects, etc.), drain the sewage in the pond, remove the sludge at the bottom, wash the pond walls, and repair the drainage pipes. Pond clearing chemicals include quicklime, bleaching powder, formalin, malachite green, etc. (1) Quicklime: when using, drain the pond water down to 10-20 cm, evenly spread quicklime, and splash it over the whole pond; the amount per mu should be 75-100 kg, and it can be used only 7-10 days after pond clearing. (2) Bleaching powder: because bleaching powder volatilizes and decomposes very easily in the air, after taking out the bleaching powder it should immediately be poured into a wooden bucket, mixed with water into a paste, and then diluted with water and splashed. If the pond has no water, the amount is 6-10 kg per mu. For pond clearing with water, apply 100 g per cubic meter of water. Bleaching powder pond clearing is carried out 4-6 days before larval hatching. (3) Formalin: has a good effect on killing viruses and protozoa in the pond. Under dry pond conditions, apply 6-10 kg of industrial formalin per mu. Splashing over the whole pond has a killing effect on Zoothamnium, and the application is carried out 10 days before larval rearing. (4) Malachite green: for pond clearing with water, make the concentration of malachite green solution in the pond water reach 0.25-0.4 ppm, which has a significant effect on killing marine zooplankton such as copepods; apply the chemical half a month before seedling rearing. (5) Trichlorfon: it is a broad-spectrum organophosphorus insecticide. Among these, dichlorvos has a strong stomach-poison and killing effect on insects. At a concentration of 0.25 ppm it has a significant effect on killing cladocerans and other organisms in the water. Apply half a month before seedling rearing. IV. Water intake and fertilization The water used for seedling rearing is filtered through bolting cloth, disinfected with 50-100 ppm bleaching powder, then subjected to sedimentation twice over 7 days, and injected into the pond after filtration through 120-mesh and 80-mesh bolting cloth. Then fertilize once a day, with an application amount of potassium nitrate 5 ppm and monopotassium phosphate 0.5 ppm. After the water color becomes light tea-colored, apply potassium nitrate 2 ppm and monopotassium phosphate 0.2-0.3 ppm daily. If the content of copper, zinc, etc. in the water exceeds water quality standards, add 3 ppm EDTA to chelate them. V. Feeding 4-5 days before the river crab larvae hatch, apply calcium nitrate 1-1.5 kg per mu, and at the same time inoculate the pond with previously cultured single-cell algae liquid. Once the larvae hatch, they can feed on the single-cell algae. If there is cloudy weather and the algae do not reproduce well, splash 1-1.5 kg per mu of cooked soybean milk to fertilize the water. When the zoea larvae enter stage II, begin feeding them rotifers and small Artemia nauplii. From stage IV zoea larvae to the megalopa stage, feed sufficient Artemia nauplii daily, making their density more than 1 times the density of the river crab larvae, to ensure a sufficient bait base.