First, understand the optimal timing for seedling handling. Newly hatched fry are still nourished by their yolk sac, have weak bodies, and a large yolk, which prevents them from swimming freely. They rely on water currents to move and must be carefully raised. Once they absorb their yolk completely, they gain some swimming ability and can then be transferred to a holding pool. This stage usually occurs 7 to 8 days after hatching, with fry reaching a body length of 10 to 14 mm.
Second, raising the seedlings requires proper facilities and methods.
1. Seedling Holding Facilities: The main tools include flowing cement tanks and cages. Cement pools can be round or square, typically ranging from 3 to 5 cubic meters in size. The water volume should not be too large to ensure better control. Inlet and outlet points must be covered with 40- to 50-mesh screens to prevent fry from escaping. Cage dimensions are usually 2 meters long, 1 meter wide, and 0.5 meters deep.
2. Raising Methods:
(1) Cement Tank Holding: Place yolk-sac fry into a flowing cement tank at a density of 25,000 to 30,000 per cubic meter. For the first 3 to 4 days, maintain continuous water flow to ensure sufficient dissolved oxygen. After 4 to 5 days, when the fry become more active and start swimming freely, introduce zooplankton or artificial micro-foods. Live foods like brine shrimp, red worms, and midge larvae are ideal. Feed them several times a day until their body color changes from reddish to dark gray. Throughout this period, keep the water gently flowing, well-oxygenated, and clean. Disinfect the tank regularly with potassium permanganate at a concentration of 8 g/m³.
(2) Cage Holding: Construct a cage using 40- to 50-mesh nylon net with dimensions of 2 meters x 0.8 meters x 0.5 meters. Place the cage in a disinfected pond 3 to 4 days before introducing fry to allow it to soften and reduce friction. The water should be clear, with over 50 cm transparency and plenty of zooplankton. Stock 5,000 to 10,000 fry per square meter of cage. Ensure the cage’s outlet is 4 to 5 cm below the water surface so fry can easily swim out. After 4 to 5 days, most fry will leave the cage naturally. If any remain, manually release them. Maintain clean, oxygen-rich water (above 5 mg/l), wash the cage regularly, and check for damage to prevent escape.
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