What is the cause of the death of fish in winter? How to prevent overwintering death?

According to data, the average mortality rate of fish species caused by improper overwintering reaches 15% every year, and the mortality rate in North China is as high as 20%, sometimes more than 30%. Because of the decrease in the number of fish species, the shortage of fish stock in the second year has a direct impact on the production of the whole year. So what is the cause of fish death in winter? How to prevent the overwintering death of fish species?

I. causes of death of fish species in winter

1. Poor conditions in fish ponds and lack of oxygen in fish species

First, the pond aging, many years of uncleared silt, a lot of organic matter, oxygen consumption, causing hypoxia; second, harmful substances exceed the standard, causing fish poisoning and death; third, the newly dug pond, the water is not fat, oxygen production capacity is poor, resulting in hypoxia death; fourth, fish pond leakage is large, poor water retention, and lack of fresh water supply, pond water shallow hypoxia; fifth, the pond surface is covered with ice, small transparency, poor plant photosynthesis caused by hypoxia.

two。 The water, fertilizer and fish are dense, and the specification is too small.

After entering the overwintering period, the fish of small size (less than 10 cm) accumulate less nutrients, which is not enough to consume during the overwintering period, and will die of weakness. If the stocking density is too high, the bad water quality will cause dead fish. In addition, if there are too many wild miscellaneous fish in the pond and consume too much oxygen, it will sometimes cause dead fish.

3. The fish was injured and died of illness

When pulling the net and merging the pond, due to the rough operation, resulting in fish injury, bacterial invasion, leading to infection, easy to die during the overwintering period.

4. Poor management

It is embodied in the following aspects:

The main results are as follows: (1) the overwintering time in the pond is too early and stop eating too early, resulting in the weight loss and physical decline of the fish in the middle and later period of overwintering.

(2) flushing water blindly or turning on aerator for a long time during overwintering, resulting in a sharp drop in water temperature, resulting in frostbite or even death of fish.

(3) the fish died due to lack of oxygen and asphyxiation due to the failure of ice breaking and snow sweeping in time.

(4) failing to disinfect and sterilize the aquaculture water or abuse drugs in time, resulting in disease or poisoning and death of the fish.

(5) during the overwintering period, the activities of oxygen and fish and the change of water level were not checked in time, or effective measures were not taken in time after finding bad conditions.

II. Methods to prevent overwintering death of fish species

1. Choose a good fish species overwintering pond

The overwintering pool should have the following conditions: first, it should be leeward to the sun. Second, the area should be suitable, generally 1500 to 2000 square meters is better. Third, the water retention capacity should be strong, the bottom material had better be clay, and the leaking pond should not be used as an overwintering pond. When using natural pits and ponds for overwintering, the bottom of the pond should be flat and less silt, and those with too much silt must be cleared and disinfected. Fourth, it is necessary to maintain a certain water depth, which is generally 2 meters, preferably more than 2.5 meters, and has supplementary water sources.

two。 Cultivate large size fish species

The cultivation of fish species must be strengthened before overwintering, and enough feed with high fat and protein content must be fed to cultivate full-bodied fish species.

3. Master the time of merging ponds

The stocking time of overwintering fish species in ponds should not be too early or too late. If the water temperature is too early and the fish is still active, the oxygen consumption is high, which is disadvantageous to the close culture in the pond. If it is too late, the water temperature is too low, and it is inconvenient to operate in the pond. Generally, the water temperature is about 10 ℃ and the pond is suitable, not less than 8 ℃.

4. Beware of fish injuries

The fish is easy to suffer from water mildew after bruising. When the fish species merge with the pond, the operation should be light and slow, so as not to damage the fish.

5. Reasonable close cultivation

Overwintering ponds with running water conditions can release 40,000 to 80,000 fish species (specification 10cm) per 667 square meters, and static water overwintering ponds with good bottom quality and supplementary water sources can be released less. The specific stocking density should be flexibly controlled according to pond conditions and fish species specifications, and excessive stocking density is strictly prohibited.

6. Fertile water overwintering

Inject green fertilizer and water (with more phytoplankton) into the overwintering pond before overwintering, or apply chemical fertilizer to cultivate phytoplankton in the overwintering pond. For long-term no water (or unable to replenish water), the oxygen content of the overwintering pond, timely topdressing of chemical fertilizer. In a pond with a depth of 2 meters, 667 square meters of ammonium nitrate 5kg and calcium superphosphate 3kg, put it in a gauze bag and hang it in water to slowly dissolve it.

7. Do a good job of keeping fish fry fat

Fish species still have to eat in windy and warm weather during the winter. Therefore, the overwintering pond should be properly fed. When it is sunny and warm, it can be fed once a day, or once every 2-3 days, and the daily feeding amount is generally 1 inch, 6 pounds, 1 pound, 8 in autumn.

8. Do well the management of overwintering

First, it is necessary to often check whether there is leakage in the overwintering pool, especially whether there is a problem with the inlet and outlet of the overwintering pool, and generally keep the water depth above 2 meters. Second, it is necessary to frequently check the water quality, water color and the activities of pond fish, and find that the water quality is uncomfortable or there is little water, and add new water in time. The amount of dissolved oxygen in water is determined regularly. When the dissolved oxygen in the pond drops to 3.0 mg / L or when you see wild miscellaneous fish competing for oxygen in the shallow water near the pond, you should add new water or start the aerator. In addition, it is necessary to patrol the pond in the severe winter, break the ice or drill holes in the pond to increase oxygen in time when the ice seal is serious, and remove the snow and sundries on the ice to maintain the light transmittance of the ice.

9. Do a good job in the prevention and control of fish diseases

During the overwintering period, fish species are easy to suffer from printing disease, water mold disease, oral filariasis and tube disease, so the fish should be washed with appropriate chemicals according to the local actual conditions before going into the pond. When the water temperature rises, quicklime or other drugs can be injected regularly to prevent fish disease. Once a fish disease occurs, it should be treated in time.