Key Technical Points for Low-level Pond Culture of Green Grouper

I. Low-level pond renovation: Select low-level ponds with an area of 2000-3335 sq m and a pond depth of 1.8 m, where the water depth can always be maintained above 1.2 m. Retain the original earth embankments around the low-level pond, convert the pond bottom to a sandy bottom or sandy-mud bottom, and set water inlet and outlet openings at both ends of the pond. For every 1000 sq m of culture area, equip 1 paddlewheel aerator. After the infrastructure renovation is completed, use quicklime at 100 kg/667 sq m to clear the pond and disinfect it, and check whether all relevant facilities are intact.

II. Seedling selection and stocking. At present, the sources of green grouper fry are natural fry and artificially bred fry. Considering factors such as the strong seasonality of catching natural fry, limited purchasing channels, and unstable quantities, the selected seedlings are artificially bred fry. 1. Selection of fry. Grouper is a fish that exhibits both cannibalism and territoriality. The purchased green grouper should preferably be uniform in body size, with intact bodies, bright body color, no external injuries, no deformities, good feeding conditions, fully spread pectoral and dorsal fins, and healthy and vigorous. When selecting smaller-sized fry, they can first be size-graded and reared before being stocked into the pond. 2. Disinfection of fry. Before stocking, the fish should be disinfected. Disinfection of fry can be carried out only 3-4 h after the seedlings arrive at the culture site. There are many commonly used disinfection methods. We use potassium permanganate at 10-20×10-6 for 5-10 min, or formalin at 20-50×10-6 for 40-10 min. 3. Stocking of fry. Place the disinfected fry into a size-grading pond for temporary holding and observation. After they are normal, stock the fry directly into the pond according to the pre-determined stocking plan. The stocking density should not be too high, because pond culture is not as convenient to operate as cage culture, and 30-50 fish/sq m is appropriate. After the Spring Festival in Hainan, the temperature is basically around 20°C, which is the best time for stocking.  III. Feed and feeding. 1. Feeding fresh trash fish and shrimp feed. Use small trash fish and shrimp as the main bait, and ensure freshness and quality. During the rearing process, appropriately adjust the feeding rate as the fry grow and as body weight and water temperature change. The feeding amount should ensure that 80% of the fish are full; generally, finishing the feed within 40 min after feeding is considered appropriate. 2. Mixed feeding of fresh trash fish and shrimp with compound feed. With the continuous development of marine fish culture, using only fresh trash fish and shrimp as bait makes it difficult to meet development requirements in terms of resource supply. Through experiments, some compound feeds for Japanese eel or European abalone were selected and appropriately mixed with small trash fish and shrimp for feeding, and no abnormalities were observed in the culture results. 3. Feeding time. According to observations, the growth of green grouper is closely related to feeding frequency. Practice has proved that feeding once every 36 h results in fast growth, high survival rate, and high yield of grouper, and can greatly reduce the feed conversion ratio. We schedule feeding at 7-8 a.m. or 5:30-6:30 p.m. before the sun goes down. IV. Daily management. Maintaining a stable and good water quality environment is something that must always be paid attention to in management. Change the water once every 20 days, with a water exchange amount of 20-30 cm. When the external water condition is good, more water can be exchanged. During water exchange, thoroughly discharge waste to remove feed residues and fish feces. After changing the water, splash a disinfectant over the whole pond for disinfection, and disinfect again every 20 h within 1-2 d. Maintain appropriate fertility and transparency of the pond water, and strictly prevent algae from growing in the pond water. In pond fish culture, mass mortality during the culture period is greatly related to hypoxia. Therefore, the culture pond should maintain a relatively deep water level, and the aerator can be turned on appropriately according to the fish carrying capacity of the pond. Generally, turn it on for 2-3 h in the second half of the night on cloudy and rainy days, and for 1-2 h at noon on sunny days. Grouper is a ferocious fish, and cannibalism is its nature. During the culture process, individual size differences should be strictly controlled. Patrol the pond frequently. When individuals are found to differ too greatly, they should be screened and reared in separate ponds to avoid unnecessary losses. The incidence of disease in grouper culture is not very high, but disease prevention work still needs to be done well. Regularly feed medicated bait alternately, conduct regular sampling inspections, and when abnormalities are found, treat with the right medicine according to symptoms. V. Harvesting in batches. Grouper cultured to more than 400 g can be sold. At the beginning, large fish can be caught while small ones are left, and harvested and sold in batches. Healthy and strong fish can be used as broodstock and polycultured with other fish species.