Detailed Explanation of Grouper Fry Nursery Rearing Technology and Daily Management Steps

1. Conditions for nursery rearing

① Site: Rearing can be done directly in the hatchery pond, or a nursery pond can be built within the culture pond. It should be a rectangular cement tank with a volume of 10-25 m3 and a water depth of 1-1.8 m.

② Lighting: Indoor nursery tanks use natural light; outdoor nursery tanks should avoid direct sunlight during the day, and foam can be used to control light intensity.

③ Water temperature: The suitable water temperature is 22-29°C, and drastic day-night temperature fluctuations should be prevented.

④ Salinity: Euryhaline, able to grow in water with salinity above 10‰. The optimal salinity is between 25-29‰.

⑤ Water environment: Keep the environment quiet and the water quality fresh. Continuous flow-through rearing is best, with balanced and normal inflow and outflow, not fluctuating. Maintain pH between 7.8-8.4 and dissolved oxygen above 5 mg/L.

⑥ Stocking density: Determined according to fry size. Larger specimens can be stocked at lower density, while smaller ones can be stocked at appropriately higher density.

2. Feed feeding

Fry above 2.4 cm can be fed minced fish and shrimp meat. Observe feeding conditions daily; when the amount is insufficient, a small amount of adult Artemia can be added as a supplement. When fry reach a total length above 3.6 cm, they can be switched to fresh chopped fish, small miscellaneous shrimp, or smaller live fry. When fry have a poor “appetite”, some artificial compound feed to aid digestion can be added as a supplement.

Whether feeding minced fish and shrimp meat, or fresh chopped fish, small miscellaneous shrimp, or smaller live fry, each kilogram of minced fish and shrimp meat and fresh chopped fish, small miscellaneous shrimp, or smaller live fry should have 10 mL of active garlic treasure and 10 g of enzymatic polysaccharide (or Jintaikang) added, mixed well and left for 1 hour before feeding. This not only protects the liver, strengthens the intestines and nourishes the stomach, promotes feeding attraction and growth, but also enzymatically decomposes parasite eggs to prevent and control parasite breeding. 3. Daily management

Maintaining stable and good water quality and substrate environment is something that must be constantly attended to in management. Every 3-5 days, apply 0.5 g each of Liukongdi Jiankang and Lilishen per cubic meter to keep the substrate and water quality fresh. During each feeding, exchange of old and new water must be carried out. At the same time, observe the feeding situation and satiation rate of the fry. Keep track of the amount of residual bait in the water body at all times so that timely adjustments can be made. Bottom cleaning work is very important, and siphon cleaning must be carried out regularly every day. If the tank is found to be dirty, change tanks promptly. Closely monitor the physiological and ecological responses of the fry. Due to different growth rates, individual differences arise among fry, and obvious differentiation within the population can cause serious mutual cannibalism. At this time, grading and rearing should be carried out as early as possible.

Fry grading: Grade once every 10-15 days; fry can be graded manually. That is, visually distinguish sizes, then select sieves of various different sizes. After screening, place them separately in different nursery tanks. Note that operations must be done very carefully to avoid damaging the fish body surface. A certain amount of medication can be added to the newly changed nursery tank to prevent infection caused by injuries from improper handling.

Grouper is a carnivorous and ferocious fish, and mutual cannibalism is its nature. There are roughly three reasons for this phenomenon: ① insufficient bait; ② large individual differences; ③ excessive stocking density. Therefore, to improve production efficiency, stocking density is a serious issue that cannot be ignored. According to observations, the rearing density after grading should be maintained at around 500-3000 individuals/m3. When fry transition to benthic life, excessive clustering of the population sometimes occurs. The main preventive method is to maintain flow-through rearing as much as possible with good water quality. If a small number of diseased fry or fry with weak vitality are found, they should be promptly removed and isolated for treatment.

4. Disease prevention and control

Since grouper fry are reared under high water temperature conditions, water quality is easily polluted by the secretion of metabolic products. Therefore, diseases are very likely to occur during the nursery rearing process. The most common diseases are vibriosis and white spot disease. According to observations, vibriosis is mostly caused by dirty tanks and generally manifests as rotting of the pelvic fins. White spot disease, however, can occur under any circumstances, mostly showing seasonality, with symptoms of rotting and white spots appearing on the head or various parts of the body. Prevention and control measures: ① Dilute Baianwei (0.5 mL per cubic meter) and splash it over the entire tank, then on the second day, mix and dry-scatter 0.5 g each of Liukongdi Jiankang + Dishuan per cubic meter; ② Mix 10 g each of active garlic treasure, Bencao Dakang, Jintaikang, and Baogan Cuzhangling per kilogram of bait and feed, 2 meals a day for 3 consecutive days. In addition, during the nursery rearing process, due to the influence of external factors, viral infections also occasionally occur. Feeding can be appropriately stopped, and at the same time, dilute Baianwei (0.5 mL per cubic meter) and splash it over the entire tank, then on the second day, mix and dry-scatter 0.5 g each of Liukongdi Jiankang and Dishuan per cubic meter.