The cause of death of river crab in temporary culture

The temporary cultivation of river crabs has two purposes: one is to make some "yellow crabs" fully mature into "green crabs"; the other is to sell them at a price to give full play to regional and seasonal price differences to obtain higher profits. However, many farmers unilaterally pursue the scale of temporary culture and ignore the improvement of temporary culture technology of river crabs, which can easily lead to the death of a large number of temporary crabs and great economic losses.

Cause of death

1. Physiological death: after the gonad matures, the crab needs to enter the salty and fresh water area at the junction of the sea and river for reproductive migration. If it continues to be temporarily raised in fresh water, it will cause excessive osmotic pressure in the body and die.

2. Poor quality of river crabs: temporary crabs with different specifications, high density and long time are easy to kill each other, and the mortality rate of crabs with small size is high; if crabs with disease and incomplete limbs are easy to cause disease transmission, temporary crabs die from the disease. Some farmers buy river crabs before temporary culture, because the river crabs are out of water for a long time, the Gill cavity and Gill filaments are seriously lost due to respiration for a long time, resulting in the death of a large number of river crabs.

3. There are many temporary cultivation links: when some temporary farmers start to catch river crabs, they first use closed cages to temporarily raise crabs, and then transfer to the soil ponds for temporary cultivation. Because the mesh of the cage is an attachment for river crabs to climb, when the density is high, it is easy to break the appendages of river crabs, and some river crabs will die due to loss of protective ability due to foot damage.

4. The ecological level is poor: some temporary adopters only focus on preventing escape and ignore the requirements of river crabs on the ecological environment. For example, the area of the temporary culture pond is too small, the coverage rate of aquatic plants is low, and there is no hidden artificial nest for the river crab, which greatly reduces the ability of the river crab to complete the last molting; the shallow water level leads to the rapid change of water quality and the low dissolved oxygen, which makes the river crab come ashore frequently, resulting in more physical exertion; the ineffective desilting of the temporary culture pond makes the water partial acid and the disease resistance of the temporary culture crab is poor; the water quality is fat and the temporary culture density is high, which will also cause the asphyxiation and death of the temporary culture crab. In this harsh environmental conditions, the above reasons lead to an increase in the mortality of temporary crabs.

5. Insufficient feeding: the river crab will still feed when the water temperature is above 4 ℃. If the amount of bait is insufficient, coupled with the lack of natural bait organisms in the temporary culture pond, it will cause river crabs to kill each other for limited bait, and some river crabs will die without a tunnel or escape because of bait.

Points for attention in temporary care

1. The choice of temporary recuperation pond. Taking into account the conditions of water source, water quality, silt and water exchange, we choose the vicinity of micro-flow water with sufficient water source, non-pollution water quality and rich dissolved oxygen, with less silt deposition, more than 40% coverage of aquatic plants, secluded places, and Tangkou, which is convenient for injection and drainage, with an area of 3 million mu and a water depth of 1.5 to 1.8 meters. The pond is thoroughly cleared half a month before the crab enters the pond, with 100kg quicklime per mu, which can not only kill pathogens, but also provide ca2+, to improve pH.

2. Requirements for temporary culture of crabs. Temporary crab culture requires strong physique, uniform specifications, complete carapace and appendages, no disease, no injury, and foot release at one time. The selected river crabs were soaked in 4% salt water for 10 minutes for 15 minutes, then sterilized. If soft-shell crabs and top-shell crabs are encountered in the process of stocking, they should be removed separately and temporarily raised in the shell after hardening, and then put into the temporary pond.

3. Reduce the temporary maintenance. It is best not to use a cage to temporarily raise river crabs. If the time for temporary cultivation is short, water peanuts can be laid around the box for temporary cultivation. Temporary cultivation density should depend on temporary cultivation conditions, management level and duration, generally controlled at 150-200kg per mu; river crabs transported for a long time should be inspected one by one before entering the pond to eliminate disabled crabs, dying crabs, dead crabs and diseased crabs, and it is best to raise them in ponds by stages to facilitate fishing and sale.

4. Prevention and control of temporary diseases. In the temporary breeding of river crabs, regularly sprinkle 0.5mg / L strong chlorine to disinfect the water body, and feed with oxytetracycline bait every semimonthly for 5 days to prevent crab disease; if there is a phenomenon of decay of crab abdominal appendages, anal redness and swelling, and refusing to eat, you can use 2mg / L oxytetracycline and crab Kangning Ⅱ bait to feed; if you find water rats, water snakes, frogs, etc., you can use drugs (capture) to kill, tools to kill, manual capture and other methods to kill.

5. Temporary maintenance management measures. If in order to make high profits, temporary breeding objects to choose green crabs will reduce the mortality rate, because they do not need to complete reproductive molting and have a strong ability to adapt to the environment. The temporary culture time of river crab is generally 1 and a half to 2 months, especially when the water area is small, the water quality is poor, the water exchange is slow, and the temporary culture density of river crab is high. After temporarily raising river crabs into the pond, the scientific management must be strengthened in time. The main purpose of daily management is to keep the water fresh. Choose to change the water at noon every 5-7 days, change water 15-25 cm each time, sprinkle quicklime every 20-30 days to increase the calcium quality of the pond water, remove residual bait, sundries and dead crabs from the pond every day, and feed once a day, the feeding amount is 3%-5% of the temporarily raised crabs, of which animal bait accounts for 60%, to ensure the healthy listing of river crabs.