Technical response measures for river crab farming hit by snow and ice disasters

I. Management of market-sized crabs:
1. Cutting ice holes and breaking ice. Market-sized crabs are held at high density in temporary storage, and temperatures are relatively low after snow, making farming management more difficult. Cutting ice holes or breaking ice serves, on the one hand, to observe whether the overwintering pond is short of water or oxygen, and on the other hand, to allow part of the harmful gases in the water to escape. When oxygen in the water is low (below 1.8mg/L), aquatic insects such as Cyclops and water mites can be seen gathering near the ice holes; measures should be taken promptly based on what is observed.
2. Aeration. 85% of the oxygen in the water comes from photosynthesis by phytoplankton, with a small part coming from dissolution from the air and from water added during replenishment. (1) Overwintering ponds should be sealed with clear ice; if dark ice forms, it should be broken so the water refreezes, ensuring clear ice covers more than 50% of the area. After snow, remove accumulated snow promptly, clearing more than 80% of the pond area. If dark ice forms or snow is not cleared in time, light under the ice is insufficient, photosynthesis is blocked, and oxygen deficiency easily results. (2) Mechanical aeration. A 1.5-3.0 kW aerator can be run continuously for 2-3 hours for aeration. When the overwintering pond is low in oxygen and short of water sources, a water pump can also be used for circulating-water aeration. Air cylinders or air pumps can also be used to force air into the water for aeration. (3) Chemical aeration. In emergencies, chemical agents such as Zengyangling can be used for aeration.
3. Water replenishment. Ensure the water depth under the ice in the overwintering pond is above 0.8 m; if it is below this level, add water at any time to fill it up.
4. Harvesting. Temperature is the key factor determining whether harvesting is possible. When above 0°C, drain the pond to catch crabs; when below 0°C, let water into the crab pond. In practice, drain the pond to catch crabs during the day and let water back into the pond in the evening.
　　
II. Management of river crab seed stock:
After snow, due to poor body condition of river crabs, low oxygen in the water, freezing and the impact of predators, poor management often leads to the death of juvenile crabs. To help river crabs overwinter safely, crab farmers can adopt the following methods:
1. Water level. The overwintering pond should maintain a water depth of about 1.5 m; only in deeper water will river crabs avoid frostbite and death. Check the overwintering pond frequently for seepage. Under no circumstances should the crab pond be left at a low water level for an extended period, otherwise, even though the river crabs hide in burrows or mud, they are still exposed to the air and will suffer frostbite or die. If the water level is found to have dropped, add water promptly, with the temperature difference of the incoming water not exceeding 2°C.
2. Feed feeding. River crabs basically stop feeding after a snow disaster. When the snow and ice melt, river crab activity increases, so pay attention to maintaining fresh water quality. When the water temperature rises to about 10°C, appropriately increase the supply of animal-based feed high in fat and protein, cooked wheat grains, corn, etc.
3. Routine management. Dedicated personnel should patrol the pond; ice on the frozen pond surface should be broken promptly; minimize disturbance to the crabs from boating, catching fish and shrimp, etc., to avoid the crabs being startled, darting about, becoming injured and expending energy.
　　
III. Management of parent crab ponds:
The harvesting, transport and temperature drop of parent crabs produce a strong sexual stimulus, keeping parent crabs at a relatively high level of excitement; the poorer the quality of the parent crabs, the higher the mortality rate, the greater the decline in egg production, and the poorer the quality of the eggs produced, often resulting in severe egg death.
1. Selection of the overwintering pond. Parent crabs generally overwinter in outdoor earthen ponds. The overwintering pond should be chosen in a wind-sheltered, sunny place close to a water source and in a relatively quiet environment, oriented east-west, rectangular or square earthen pond, with a water depth above 1.5 m, and soil preferably sandy loam or clay.
2. Water quality regulation. During the overwintering period of egg-bearing parent crabs, when the water body is frozen, insert an appropriate amount of grass bundles, break the ice every morning before the sun rises, and promptly sweep away snow on the ice. The key is to maintain the relative stability of the water environment; its main water quality indicators should be monitored once every half month, with salinity around 25‰, dissolved oxygen above 5mg/L, and pH preferably 7.8-8.7. For leaking earthen ponds, add water once every 5-7d, and each water exchange should not exceed 30% of the total water body, so as to maintain the water level above 1.5 m. When the temperature reaches above 5°C, fresh seawater should be added and replaced at regular intervals. This is because when the water temperature is above 5°C, although the eggs (embryos) of the parent crabs develop very slowly and there are almost no metabolic products, the metabolic capacity of the parent crabs themselves begins to increase; added to the relatively high density of farmed parent crabs, water quality will gradually deteriorate and affect the embryonic development of the parent crabs, so water quality regulation should begin at this time.
3. Feed feeding. Choose nutrient-rich live feed, such as sandworms and fresh mixed fish. After parent crabs carry eggs, as the water temperature keeps changing, the daily feeding amount is generally controlled at 2-10% of the total body weight of the parent crabs, depending on their feeding situation. Regardless of how the surface water temperature changes, as long as parent crabs are found to be active, they should be fed appropriately. Feed once a day before sunset, placing the feed below the water level around the earthen pond; the next day patrol the pond to check the feeding situation of the parent crabs, remove leftover bait, and adjust the feeding amount accordingly. Multiple bait types should be fed alternately. When the water temperature is above 8°C, fresh bait and compound feed should be fed. When the water temperature rises above 8°C, the embryos of the parent crabs begin to accumulate temperature, development gradually accelerates, and the physical energy consumption of the parent crabs increases. First, the parent crabs continuously fan their tail fans to make the water around the embryos flow, carrying away metabolic products and obtaining dissolved oxygen, promoting embryonic development; second, the parent crabs may continuously secrete a kind of "maternal hormone" to protect the normal development of the embryos. At this time, in addition to regulating water quality well, it is also necessary to increase the feeding of some fresh bait and high-quality compound feed to meet the physiological needs of the parent crabs and ensure they have enough nutrition and physical strength to reproduce.
4. Routine management. Patrol the pond once each morning and evening to observe the activity of the parent crabs, their feeding situation and changes in water color, etc.; check whether the escape-prevention facilities are damaged; every half month, regularly examine under a microscope the development of the fertilized eggs of egg-bearing parent crabs, and promptly adopt and adjust management measures to ensure that egg-bearing parent crabs overwinter smoothly.
　　
IV. Key points for market-sized crab production after the disaster:
1. Pond clearing and preparation. Before river crabs are stocked, the pond must be cleared and prepared; after snow, the pond embankments should also be reinforced and excess silt removed. About l5 days before crab seed stocking, thoroughly clear the pond; there are 2 methods of pond clearing: the dry method and the water-retained method. Generally, the dry method uses l000 kg of quicklime per hectare, which is dissolved and splashed over the whole pond. The water-retained method is used only when most of the pond water cannot be drained; the specific amount can be flexibly controlled according to the depth of the pond water, as long as it can ensure the killing of harmful organisms in the water.
2. Reinforce and repair escape-prevention facilities. Firm escape-prevention equipment must be built around river crab farming ponds to prevent river crabs from escaping. The inner wall of the escape-prevention equipment should be smooth; commonly used materials include plastic film, iron plates, aluminum plates, etc.
3. Protect aquatic plants from freezing. Some farmers have already planted aquatic plants such as Elodea; after snow, when it freezes, prevent the aquatic plants from freezing to death. The specific method is to ensure the water level is higher than the aquatic plants, so that the ice layer is above the aquatic plants.
4. Postpone stocking crab seed. When the temperature is below 0°C, crab seed must not be harvested, transported or stocked, to prevent the crab seed from frostbite. Wait until the weather warms up and the temperature is above 0°C before carrying out operations related to crab seed.