Sharing Experience on Overwintering and Broodstock Preservation of Tilapia in Outdoor Ponds

Currently, most tilapia overwintering methods use indoor ponds, which require demanding conditions and allow only a limited number of overwintering fish. In order to further increase the number of overwintering fingerlings, the following introduces outdoor pond overwintering and broodstock preservation technology:

1. Overwintering facilities: outdoor pond, area 1-1.5 mu, depth 1.7-1.8 m, water depth 1.5-1.6 m, preferably with stone-lined walls and bottom, with bottom mud not exceeding 10 cm, the less the better. At one end of the pond, install 1-2 water level balance pipes (1.5 m from the pond bottom) and 1-2 drainage pipes (0.5 m from the pond bottom), all leading to the drainage ditch; one hot water pipe and one cold water pipe (placed close to each other; it is even better if the hot and cold water are mixed well before being injected into the overwintering pond); two 1.5 kW paddlewheel aerators (or one paddlewheel type and one waterwheel type).  2. Water quality conditions: The chemical indicators of the water quality should conform to the water quality standards for freshwater fisheries. In addition, dissolved oxygen and water temperature should be determined according to the size of the fingerlings stocked and the required size at harvest. That is, if the fingerlings are required to grow considerably during the overwintering period, dissolved oxygen and water temperature should be appropriately higher; conversely, if the purpose is only broodstock preservation, dissolved oxygen and water temperature can be appropriately lower.  3. Fingerling stocking: Around National Day, stock the fingerlings. Before stocking, thoroughly clear the pond with quicklime. The stocking density depends on the size of the fingerlings stocked and the required size at harvest; that is, if the fingerlings are required to grow substantially after overwintering, the stocking density should be lower; conversely, if the purpose is only broodstock preservation, the stocking density can be higher, in the range of 5-10 kg/sq m.  4. Feed requirements: The density of overwintering fingerlings is high, and natural food is insufficient, so during the overwintering period, high-quality artificial compound pellet feed should be fed according to the size of the pond fish. The pellet size should match the mouth size of the fish to ensure feed palatability.  5. Water quality regulation: There are three keys to water quality regulation for high-density overwintering and broodstock preservation: dissolved oxygen, water temperature and waste. Dissolved oxygen is regulated through aerators, while water temperature and waste are regulated through water exchange.  Dissolved oxygen: If the stocked fingerlings are small in size and low in weight, then after stocking, one paddlewheel aerator should be placed in the pond, with operating time controlled at 30 minutes/hour. As the fish grow, the operating time should be increased to 50 minutes/hour. According to the growth of the pond fish, another aerator should be added in due course, with both running simultaneously for 40 minutes, then one stopped while the other continues working; after 20 minutes, both run simultaneously again, and after 40 minutes the other is stopped for 20 minutes. Rotate operation in this way repeatedly. After April, the two aerators should basically remain in operation day and night without stopping.  Water temperature: Warm water is added through the inlet pipe, and through the action of the aerators, the pond water is mixed evenly. Since the drainage pipe is generally not opened, as warm water is added, the excess pond water flows out through the water level balance pipe. Due to the addition of warm water and the outflow of the original pond water, the pond water can be controlled at a certain temperature.  Waste: The fish population is large, metabolic products are abundant, and together with the effect of residual bait, the pond water deteriorates relatively quickly. Although the water level balance pipe can discharge some waste, the amount is limited, and a large amount of waste still needs to be discharged through the drainage pipe. When discharging waste, open the drainage pipe valve to lower the pond water level to 0.5 m, then close the valve and add water to restore the water level to 1.5 m; another method is to open the valve to lower the water level to 0.5 m, add water to the pond to stir up the sediment in the pond, let it flow out through the drainage pipe with the water, then close the valve and restore the water level to 1.5 m. Through frequent waste discharge, waste in the pond water will not accumulate excessively.  6. Feeding management: After the fingerlings are stocked, start feeding compound feed six times a day, at: 7:00-8:00, 9:00-10:00, 11:00-12:00, 13:00-14:00, 15:00-16:00, 17:00-18:00.  The feeding method is that after the fingerlings are stocked, before each feeding, strike an object to make a sound, then feed the feed. Within a week, the pond fish can form a conditioned feeding reflex. The feeding amount is determined according to the feeding situation of the pond fish: feed more if they eat more, feed less if they eat less, ensuring the pond fish are about 70-80% full, which both meets the physiological needs of the fish school and improves feed utilization.  7. Disease prevention and control  Use a combination of oral medication and external medication to prevent and control fish diseases. Prevention: Oral medication: The administration method is to make the drug into medicated bait and feed it at fixed times. There are two kinds of drugs: Chinese herbal medicine and antibiotics. The dosage of Chinese herbal medicine is 5 g per 10 kg of fish, once a month, for three days each time; the dosage of antibiotics is 4 g per 100 kg of fish, once a month, for three days each time. The two drugs are used alternately, half a month apart. External medication: The method of use is whole-pond splashing. The types include: quicklime, copper sulfate and ferrous sulfate mixture (5:2), bleaching powder, and Weidao brand disinfectant. The dosages used are 30 ppm, 0.5 ppm, 1 ppm and 0.5 ppm respectively, using one type every 10 days in rotation (because bleaching powder and Weidao brand disinfectant have similar effects, only one of them is used in each rotation). Treatment: During the overwintering period, the main fish disease that occurs is trichodiniasis, followed by Glossatella disease. The treatment measure is to splash dissolved copper sulfate and ferrous sulfate mixture (5:2) into the pond in the morning, and that evening carry out a large water exchange in the pond by opening the drainage pipe to lower the pond water to 0.5 m, then adding fresh warm water to restore the pond water. This achieves a relatively good therapeutic effect on the fish disease. As for the concentration of the mixture used, because it is frequently used during disease prevention, the pathogens develop a certain resistance to the drug, so when treating diseases, the concentration used should be appropriately increased above the usual concentration according to actual conditions. 8. Overwintering results: Through overwintering management, each square meter of water can produce about 15 kg of overwintering fingerlings, with a size of about 50 g/fish, a survival rate above 95%, and a net income of more than 100 yuan per square meter of water.