Causes of and Preventive Measures Against Winter Mortality of Fingerlings

According to available data, the average mortality rate of fingerlings caused by improper overwintering reaches 15% each year, with mortality in North China reaching as high as about 20%, and sometimes even exceeding 30%. The reduction in the number of fingerlings leads to insufficient stocking numbers the following year, directly affecting annual production. So what are the causes of winter mortality of fingerlings? How can overwintering mortality of fingerlings be prevented?

I. Causes of Winter Mortality of Fingerlings

1. Poor pond conditions and oxygen deficiency in fingerlings

First, aging ponds that have not been desilted for many years contain large amounts of organic matter that consume much oxygen, causing oxygen deficiency. Second, excessive harmful substances cause fish poisoning and death. Third, newly dug ponds have water that is not fertile and have poor oxygen-producing capacity, resulting in death from oxygen deficiency. Fourth, fish ponds with serious seepage and poor water retention, and lacking fresh water replenishment, have shallow water and oxygen deficiency. Fifth, pond surfaces covered by ice have low transparency, and poor plant photosynthesis causes oxygen deficiency.

2. Fertile water and high fish density with undersized fingerlings

After entering the overwintering period, undersized fish (less than 10 cm) store fewer nutrients, which are insufficient for consumption during overwintering, and they die from weakening. Excessive stocking density causes water quality to deteriorate and leads to fish deaths. In addition, if there are too many wild and miscellaneous fish in the pond, oxygen consumption is excessive, which can sometimes also cause fish deaths.

3. Fish body injuries and death from disease infection

During netting and pond consolidation, rough handling causes injuries to fish bodies, allowing pathogens to invade and leading to disease infection, making them prone to death during the overwintering period.

4. Poor management

This is specifically manifested in the following aspects:

(1) Consolidating ponds for overwintering too early and stopping feeding too early, causing fish bodies to become emaciated and their constitution to decline in the middle and late stages of overwintering.

(2) Blindly circulating and flushing water or running aerators for long periods during overwintering, causing water temperature to drop sharply, resulting in frostbite or even death of fish.

(3) Failure to break ice and sweep snow in a timely manner, causing fish to suffocate and die from oxygen deficiency.

(4) Failure to timely disinfect and sterilize the aquaculture water body or abuse of drugs, causing fish to become diseased or die from poisoning.

(5) Failure to timely check oxygen levels, fish activity and water level changes during the overwintering period, or failure to take effective measures promptly after discovering adverse conditions.

II. Methods for Preventing Overwintering Mortality of Fingerlings

1. Select good overwintering ponds for fingerlings

Overwintering ponds should meet the following conditions: First, they should be sheltered from wind and face the sun. Second, the area should be appropriate, generally 1500-2000 sq m is best. Third, water retention performance should be strong, and the bottom substrate should preferably be clay; leaking ponds are not suitable as overwintering ponds. When using natural pits and ponds for overwintering, the pond bottom should be flat with little silt, and those with excessive silt must be cleared and disinfected. Fourth, a certain water depth should be maintained, generally around 2 m, preferably reaching 2.5 m or more, with a supplementary water source.

2. Cultivate large-sized fingerlings

Before overwintering, fingerling cultivation must be strengthened by feeding sufficient feed with high fat and protein content, so as to cultivate plump and well-built fingerlings.

3. Master the timing of pond consolidation

The timing for consolidating and stocking overwintering fingerlings should not be too early, nor too late. If too early, water temperature is high, fish activity is still relatively strong, and oxygen consumption is high at this time, which is unfavorable for dense culture after pond consolidation. If too late, water temperature is too low, and handling for pond entry is inconvenient. Generally, pond consolidation at a water temperature of around 10°C is suitable, and it should not be lower than 8°C.

4. Guard against fish body injuries

After fish bodies are scratched, they are prone to saprolegniasis. When consolidating fingerlings into ponds, operations should be gentle and slow to avoid injuring fish bodies.

5. Reasonable dense culture

Overwintering ponds with flowing water conditions can stock 40,000-80,000 fingerlings (size 10 cm) per 667 sq m. Static water overwintering ponds with good bottom substrate and supplementary water sources can stock fewer. The specific stocking density should be flexibly controlled according to pond conditions and fingerling size, and excessive stocking density is strictly prohibited.

6. Overwintering with fertile water

Before overwintering, inject green fertile water (with more phytoplankton) into the overwintering pond, or apply chemical fertilizer in the overwintering pond to cultivate phytoplankton. For overwintering ponds that have not been replenished with water for a long time (or cannot be replenished) and have very little oxygen content, apply chemical fertilizer in a timely manner. For ponds with a water depth of 2 m, use 5-6 kg of ammonium nitrate and 3-4 kg of superphosphate per 667 sq m, put them into gauze bags, hang them in the water, and let them dissolve slowly.

7. Do a good job of maintaining fingerling body condition

During the overwintering period, fingerlings still eat on windy and warm days. Therefore, the overwintering pond should be appropriately fed some feed. On clear and warm days, generally feeding once a day is sufficient, or once every 2-3 days, and the daily feeding amount is generally 1/6-1/8 of that in autumn.

8. Carry out overwintering management well

First, frequently check whether the overwintering pond has water leakage, especially whether there are problems with the inlet and outlet of flowing-water overwintering ponds, and generally maintain water depth above 2 m. Second, frequently check water quality, water color and fish activity in the pond. If unsuitable water quality or insufficient water is found, add new water in a timely manner. Regularly measure the dissolved oxygen content in the water. When the dissolved oxygen content in the pond drops to 3.0 mg/L, or when wild miscellaneous fish are seen gathering in shallow water at the pond edge competing for oxygen, new water should be added or the aerator should be turned on. In addition, during severe winter, patrol the pond persistently. When ice sealing is severe, break the ice or make holes in the pond in a timely manner to increase oxygen, and clear snow and debris from the ice surface to maintain the light transmittance of the ice layer.

9. Do a good job of fish disease prevention and control

During the overwintering period, fingerlings are prone to print disease, saprolegniasis, ichthyobodo disease and Chilodonella disease, among others. Therefore, before fingerlings are put into the pond, appropriate drugs should be selected according to local actual conditions to soak and wash the fish bodies. When water temperature rises, quicklime or other drugs can be applied regularly to prevent fish diseases. Once fish diseases occur, they should be treated in a timely manner.