Tilapia Farming: A Detailed Guide to Overwintering and Broodstock Preservation Techniques in Greenhouses

Tilapia is a tropical fish with relatively poor cold resistance. Generally, when the water temperature drops to 12°C, tilapia will suffer frostbite or die. Oreochromis niloticus × O. aureus hybrids have relatively stronger cold resistance, but they still cannot overwinter in low-temperature environments for extended periods. In China, apart from Hainan, Taiwan, China, and parts of Yunnan and Guangdong provinces where natural overwintering is possible, tilapia in most regions cannot overwinter in natural conditions and must adopt corresponding overwintering and broodstock preservation measures to ensure the smooth overwintering of parents and fry.

There are many overwintering methods for tilapia. Based on differences in local climate and overwintering conditions, the main methods include film-covered large-pond greenhouse overwintering, hot spring water overwintering, deep wells, factory waste heat, boiler heating, and electric heater heating. As a tilapia fry production base, our facility has basically adopted a combination of film-covered large-pond greenhouses and iron-frame film-covered overwintering based on years of overwintering experience. Practice has proven this to be a relatively successful overwintering technique.

Preparation Before Overwintering

1. Selection and Preparation of Overwintering Ponds

Overwintering ponds should be selected on higher terrain with good water retention, sheltered from the wind and facing the sun. The area should not be too large, generally within 6 mu is suitable, oriented east-west, with span not too large. Half a month before overwintering, the pond water should be drained dry, and debris fish, weeds, and miscellaneous objects at the pond bottom should be removed. Pond embankments and drainage outlets should be strictly repaired, especially drainage outlet filter screens. Where conditions permit, the large pond can be sun-dried until the bottom cracks, then refilled with 10-15 cm of water for disinfection. Quicklime and tea seed cake can be used in combination for disinfection, covering the pond bottom and embankments to thoroughly kill debris fish and bacteria. Fresh external river water or well water is best for refilling. Overwintering pond water does not require fertilizing, as nutrient-rich water is not conducive to management during the overwintering period and increases management difficulty. After disinfection and refilling, fish can be test-released after 5-7 days.

2. Preparation and Construction of Overwintering Materials

The main task is building the greenhouse arch frame. A row of support frames is erected with wooden stakes in the middle of the large pond, then small steel cables are strung on top. Wooden stakes are used to secure the pond edges. After covering with film, steel wire ropes are pressed on top, and small iron wires are used to secure the upper and lower steel cables. When covering the film, it should be pulled flat to prevent rainwater from accumulating on top of the greenhouse. During the overwintering period, the film should be frequently checked for holes, and corresponding measures should be taken promptly.

3. Preparation of Overwintering Equipment

Depending on the overwintering method, equipment needed during the overwintering period such as heaters, boilers, temperature controllers, water pumps, and aerators should be prepared before overwintering.

Preparation Before Overwintering Fish Are Stocked

1. Pre-Overwintering Fattening

One month before overwintering, fish to be overwintered should be concentrated in dedicated ponds for intensive cultivation to promote plump and strong bodies, enhance cold resistance during overwintering, and gradually adapt them to the living environment during the overwintering period. Some weak or injured fish should be culled in advance.

2. Selection and Retention of Overwintering Fish

There are two types of overwintering fish: broodstock and fry. (1) Broodstock: When broodstock enter the pond, they must be strictly selected according to the requirements for fry production to ensure every broodfish meets the standards. The female-to-male ratio of broodstock is selected at 3:1 or 4:1, and males and females are overwintered in separate ponds to facilitate pairing operations during crossbreeding the following year. The number retained should be based on the planned fry production plus an additional 15%-20% to ensure the smooth progress of fry production the following year.

(2) Fingerlings: Overwintering fingerlings of 4-6 cm are preferred. Generally, fry for overwintering should be stocked at the same size. Too large will reduce the utilization rate of the overwintering pond, while too small fingerlings have poor adaptability during overwintering, low survival rates, and will result in uneven sizes of overwintering fry in the same pond. Selected fingerlings should also be robust, free of injury and disease, with smooth body surfaces and no frostbite.

Whether selecting broodstock or fingerlings, operations must be gentle, quick, and careful to avoid bruising the fish. They should be selected and stocked immediately. When grading and sieving fingerlings, they should not be crowded in net enclosures for extended periods. Generally, after hanging in net enclosures for two hours, they can be graded, counted, and stocked. The maximum time should not exceed five hours, and high-density long-distance transport should be avoided, as this will cause severe injury to the fish and result in low overwintering survival rates.

3. Fish Disinfection

During overwintering, broodstock and fingerlings suffer varying degrees of injury during transport and handling. Before entering the pond, fish should be disinfected with medication. A 2%-3% salt solution (non-iodized) can be used to soak the fish for 5-10 minutes before stocking. After all fingerlings are stocked, 0.3 ppm chlorine dioxide can be used for whole-pond disinfection to prevent disease. Within one week after stocking, close attention should be paid to the activity of stocked fingerlings, especially whether wounds from handling at lower water temperatures have become infected, and corresponding measures should be taken promptly. Only after one week of stocking do fingerlings basically stabilize, after which they enter the overwintering management period.

Timing and Density of Stocking Overwintering Fish

1. Stocking Time

Stocking should be done when water temperature is 18-20°C, and should be completed before the first cold wave. Once water temperature drops below 16°C, captured fish cannot be used as overwintering fingerlings because the water temperature will cause frostbite, and they will die gradually after stocking. When catching and stocking fish, choose stable weather with strong sunlight. Generally, seining should not begin until after 9:30 AM and should end before 4:30 PM, as water temperature is higher during this period, which is favorable for operations.

2. Stocking Density

Stocking density for overwintering fish depends on overwintering environmental conditions, fish size, and management level. In film-covered large overwintering ponds with static water and aeration, generally 5-8 kg of broodstock can be stocked per cubic meter of water. However, since broodstock need pre-spawning intensive cultivation in the original pond during the late overwintering period, facilities with conditions should reduce broodstock stocking density, generally 500-600 fish per mu at 500-600 g per fish. If pairing and spawning are to be conducted in the original overwintering greenhouse, factors such as the number and weight of female and male broodstock must also be considered. Stocking density for overwintering fry is 100,000-120,000 per mu for 3-4 cm size; 80,000-100,000 per mu for 4-5 cm size; and 70,000-80,000 per mu for fry above 5 cm. Small-size fry have lower survival rates due to their small size, so stocking quantities should be appropriately increased to reach normal overwintering levels after accounting for survival rate, and they can reach 5-7 cm size when taken out after intensive cultivation. For large-size fry, since survival rates after stocking are higher, density should be increased to maintain uniform size, with 5-7 cm being the most suitable and stable size.

Feeding and Management During the Overwintering Period

The overwintering period for tilapia is relatively long, generally from mid-to-late October to late March of the following year, about half a year. In production, overwintering is generally managed in three stages. In the early stage after stocking and the period before removal when water temperatures are higher, more feed should be given and water quality adjusted. In the middle stage when temperatures are lower, feed should be appropriately reduced to prevent water quality deterioration. Management in the late overwintering stage is particularly important. Before removal, the quantity and quality of feed should be increased to promote physical recovery and gonadal development of broodstock, thus ensuring the quality of fry after removal. Throughout the overwintering period, dedicated personnel should be assigned, records kept, and work on water temperature control, water quality adjustment, feeding, and disease prevention should be carried out.

1. Water Temperature Control

Within 10 days of stocking overwintering fish, water temperature should be controlled between 20-25°C, which helps fish wounds heal quickly and inhibits saprolegniasis. After conditions basically stabilize, water temperature can be controlled between 18-20°C. Water temperature must not fluctuate between high and low, must not drop suddenly, and must not be kept above 20°C for extended periods. Prolonged high temperatures are not conducive to overwintering management and will increase feed input, because at higher water temperatures fish are more active, consume more energy, and eat more. If ultra-thin film greenhouses are used, since sunlight raises the temperature during the day, the air temperature inside the greenhouse can sometimes reach 30°C. At this time, part of the film should be rolled open to allow air circulation, keeping the greenhouse air temperature at about 21°C and water temperature at 18-19°C. If weather changes suddenly or a cold wave arrives, the film should be promptly covered back to prevent sudden temperature drops.

2. Water Quality Adjustment

Overwintering ponds should maintain fresh water quality with sufficient dissolved oxygen. Due to the large area of overwintering ponds, water exchange is relatively difficult, so under normal circumstances water is not changed, only some new water is added. During the overwintering period, water quality can be adjusted and water vitality increased by regularly using microbial preparations (such as Liushengsu, Yangzhibao, etc.). When water color becomes too dense, part of the old water can be exchanged and new water injected, but water exchange should not be excessive, controlled at 1/4 to 1/5. When adding water, the flow should enter the pond smoothly and directly, and the outlet must be fitted with a filter screen to prevent weeds from entering or fingerlings from swimming out against the current. Daytime flushing should not exceed four hours, especially for fry ponds, which should be controlled at about 2-3 hours, to prevent fry from consuming excessive energy by swimming against the water for extended periods or from being crowded for too long, which increases body damage.

3. Aeration Equipment

Since large-pond winter greenhouse overwintering generally has relatively high stocking density and relatively nutrient-rich water, all overwintering ponds should be equipped with aeration equipment. In low-temperature seasons, because surface water temperature is lower, fish that surface for oxygen after floating to the top are extremely susceptible to frostbite and saprolegniasis, leading to ulceration and death. In the late overwintering period, as water temperature rises and feeding increases, water quality becomes nutrient-rich during pre-spawning cultivation of broodstock, causing severe oxygen deficiency and affecting cultivation. Therefore, aeration during the overwintering period is very important.

4. Rational Feeding

Tilapia should be appropriately fed nutrient-rich concentrated feed during the overwintering period. Generally, the method of more at both ends and less in the middle is adopted, meaning more feed is given for a period after stocking, with broodstock controlled at about 2% and fingerlings at about 5%-6%, fed once daily, with the ideal being that all fish can eat. For fry ponds, arrange one to two excessive feedings per week. The purpose is that some weak and small fry, which normally cannot compete for feed due to low daily feeding amounts, can eat once or twice a week to maintain energy consumption and avoid dying from weakness. Fingerling feed can be powdered or small-diameter compound feed. When feeding, distribute evenly across the entire pond so most fry can eat. Broodstock feed should be made into floating pellet feed. Sinking feed should be set appropriately and ideally consumed within 1.5 hours. If not fully consumed, residual feed should be removed promptly and feeding amounts reduced to prevent feed spoilage affecting water quality. During the overwintering period, the quantity and quality of feed should be adjusted at any time according to water temperature, water quality, and fish feeding conditions.

Daily Management

1. Monitor Water Temperature Changes

Measure water temperature and air temperature frequently, generally twice daily, and keep records.

2. Persist in Pond Inspections in the Morning, Noon, and Evening

Check whether the overwintering greenhouse is sturdy, frequently observe fish activity and feeding conditions, monitor water quality, and where conditions permit, regularly monitor indicators such as dissolved oxygen, ammonia nitrogen, and organic matter in the pond. Keep records, promptly identify problems, take measures, and prevent accidents.

3. Pay Attention to Ventilation and Air Exchange

In the late overwintering period, as water temperature rises, attention should be paid to ventilation and air exchange in the overwintering greenhouse. Once water temperature stabilizes, the greenhouse can be opened to cool down and new water added. When the water temperature in the overwintering greenhouse equals that of the outside, preparations before removal can begin.

4. Pay Attention to Disease Prevention

During the overwintering season, because overwintering ponds are small in area, high in density, relatively poor in water quality, and low in water temperature, combined with fish being less active and eating less, tilapia are more susceptible to disease. Fish disease control should mainly follow the principle of prevention first, combined with treatment. During the overwintering period, use quicklime once every half month at 10-15 kg per mu. After 7-10 days of using quicklime, use 0.25-0.3 ppm chlorine dioxide for disinfection. Use insecticide medication once a month, and note that various medications should be used separately and alternately. Frequently check fish activity, and if abnormalities are found, handle them promptly.