1. Cultivation Ponds and Stocking Density
It is recommended to use square or rectangular cement tanks with a volume ranging from 25m³ to 100m³ for the rearing of oyster larvae. The water level should be maintained, and air pipes with valves should be installed to ensure proper aeration. A drain outlet should be placed at the bottom of the tank, leading outside the pool for efficient drainage. Before starting the cultivation, the pond must be thoroughly disinfected using potassium permanganate or sodium hypochlorite, followed by exposure to direct sunlight to eliminate any potential pathogens. The stocking density is typically set between 10,000 to 20,000 larvae per cubic meter, depending on the stage of development and available resources.
2. Water Quality Management and Monitoring
During the larval rearing process, sand-filtered seawater is used as the primary water source. In the early stages, UV disinfection can be applied to further ensure water purity. Throughout the entire culture period, water quality parameters are monitored twice daily. Key indicators include water temperature (18–20°C), dissolved oxygen (≥4 mg/L), and total ammonia nitrogen (≤1.0 mg/L). Maintaining these conditions is crucial for the health and survival of the larvae.
3. Light Control and Algae Supplementation
To control light intensity, an adjustable shade screen is placed above the tank, keeping the illumination below 2000 lux. During the early stages, Chlorella algae are introduced into the water to sustain rotifer populations, prevent starvation, and maintain a stable feeding environment. The concentration of Chlorella is kept between 500,000 and 1,000,000 cells/ml, which supports the nutritional needs of both the rotifers and the larvae.
4. Feeding Regimen and Nutritional Enhancement
The feeding sequence for true oyster larvae includes rotifers, Artemia nauplii, and later, fish and shrimp meal. Each stage requires careful management to ensure optimal growth:
(1) Rotifers are introduced when larvae are about 3 days old, with a density of 3–5 mlâ»Â¹. As larvae grow to 7–8 mm, the feeding frequency increases to maintain adequate food supply. Rotifers are fed until the larvae reach around 12 mm in length.
(2) Artemia nauplii are added when larvae are approximately 6.5 mm long, initially at 0.5–1 mlâ»Â¹. As the fry grow, the amount is gradually increased. When large quantities of Artemia are consumed, supplementary feeds such as cephalopods, fish eggs, or compound feed should be introduced.
(3) Fish and shrimp meat are introduced when larvae reach 8–9 mm. The food should be finely minced and offered in small amounts several times a day. Care must be taken to monitor water quality and avoid oxygen depletion. Vitamins should also be added during this phase.
(4) Nutritional enhancement of live feeds is essential. Rotifers and Artemia nauplii often lack sufficient levels of EPA and DHA, which are critical for larval development. To address this, rotifers are enriched with chlorella and squid liver oil for 12 hours before feeding. Similarly, Artemia are enriched with emulsified fish oil, and fat-soluble vitamins and lecithin may be added to enhance their nutritional value.
5. Aeration and Opening Period Management
Adequate aeration is vital, especially during the opening period. Air stones are placed at intervals of 1.5 to 2.0 m² to ensure even distribution of oxygen. During the recovery phase, a gentle water flow (50 ml/min) helps prevent bubble ingestion and false openings. Larvae should be strong enough to complete the transition, requiring high-quality initial feed. Measures to improve survival include controlled aeration, clean water, nutrient-rich bait, and selecting robust newly hatched larvae.
6. Additional Management Practices
(1) Water Exchange: Initially, the tank is filled to 75–80% capacity. Over the first five days, water is gradually added. At day 5, a microfluidic water change is performed at 20%, increasing to 50% at day 10, 100% at day 20, and up to 200% when transitioning to fish and shrimp meal. Water exchange rates continue to increase accordingly.
(2) Circulation System: Once larvae reach 6.5 mm in length, a circulation plate is introduced to promote better water flow and metabolism. The speed is adjusted as the fry grow.
(3) Bottom Cleaning: Feces, leftover food, and dead organisms accumulate at the bottom after 12 days. Regular cleaning is necessary, with frequency decreasing as the larvae grow.
(4) Isolation of Weak Fry: High-density rearing can lead to size differences. When larvae reach 10–15 mm, larger individuals may attack smaller ones, causing injuries. These weak fry should be isolated to prevent disease spread.
(5) Counting Techniques: For larvae under 8 mm, a column sampler is used for accurate counting. After 8 mm, counting becomes more challenging, so sampling from the bottom is preferred. Finally, when removing from the tank, small nets are used to ensure precise numbers.
(6) Harvesting and Transferring: Under ideal conditions (18–20°C), larvae can be harvested after about 50 days, reaching 25–30 mm in length. At this stage, they are ready for transfer to nursery ponds. The harvesting method involves reducing the water level and gently collecting the fry with a soft net.
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