First, artificial nursery techniques are essential for the successful breeding of shellfish.
1. Selection of Pro-Shellfish
Healthy individuals aged 1 to 2 years should be chosen as broodstock. These shellfish should have intact shells, be double-shelled, with strong and tightly closed shells, sharp edges, and a plump appearance. Only those that show good vitality and physical condition should be selected for breeding.
2. Artificial Spawning and Insemination
Currently, fertilized eggs are mainly obtained through artificial spawning and insemination. The ratio of male to female broodstock is typically (50–100):1. Mature females have a slightly yellowish soft tissue, while males have milky white soft tissue, making them easy to distinguish by eye. The process involves squeezing the eggs into a bucket containing about 180 liters of seawater. Once the egg concentration reaches a suitable level, an appropriate amount of semen is added and thoroughly mixed. Any foam on the surface is removed. After 10 minutes of incubation, the mixture is filtered through a 200-mesh sieve and transferred to a hatching tank.
The hatching density is generally around 150–250 kg per 10 tons of water, with the density controlled at approximately 30 eggs per milliliter.
3. Larval Cultivation
Fertilized eggs develop into "D"-shaped larvae within about one day (at a water temperature of around 23°C). At this stage, larvae can be selected and cultured in separate tanks.
4. Feeding and Algae Cultivation
Pacific oyster larvae are fed with various microalgae such as *Phaeodactylum tricornutum*, *Nitzschia clostridium*, *Chaetoceros*, *Isochrysis galbana*, *Platymonas*, and *Chlorella*. Currently, *Chlorella* (cloud microalgae), *Nisslia spp.*, and *Nitzschia clostridium* and *Chaetoceros sp.* are commonly cultivated. These algae are usually grown directly from seawater and are collectively referred to as diatoms.
(1) Cultivation of Cloud Microalgae
Cloud microalgae is a type of green algae with small cells (1.6–2.6 microns). It has wide adaptability, grows year-round, and is a primary food source for Pacific oyster larvae. The initial inoculation density is 3.5–4 million cells per milliliter. As the cell count increases, fresh seawater must be added regularly to prevent aging. Typically, 1/3 to 1/2 of the volume is replaced each morning, maintaining a density of around 4 million per milliliter. At noon, a small amount of water is added to keep the density below 7 million per milliliter. Fertilization is done every other day, using 5 g/m³ nitrogen (N), 0.5 g/m³ phosphorus (P), and 0.025 g/m³ iron (Fe).
(2) Cultivation of Seawater Diatoms
Diatoms are collected directly from seawater using a 300-mesh sieve. In areas like Luoyuan Bay, where water temperatures are below 25°C, *Nitzschia clostridium* (commonly known as small diatoms) dominate. When the temperature rises above 25°C, *Chaetoceros* becomes more prevalent. The initial inoculation density is 152,000–200,000 cells per milliliter. Due to rapid growth, continuous water addition is necessary to maintain a density under 400,000 cells per milliliter. The tank should be flushed every other day to ensure continuous culture. Each morning, the fertilizer is calculated based on the total volume of water. The recommended application rates are 3 g/m³ nitrogen (N), 0.3 g/m³ phosphorus (P), 0.015 g/m³ iron (Fe), and 0.3 g/m³ silicon (Si).
5. Larval Density Management
The ideal density for "D"-shaped larvae is 4–5 per milliliter. During early stages, the density should be around 2–3 per milliliter, and during the eye-stage, it should be reduced to 1 per milliliter.
6. Feeding Practices
Larvae are primarily fed with cloud microalgae and seawater diatoms. As the larvae grow, the amount of feed should be increased. After feeding, the gastrointestinal fullness of the larvae should be monitored. Most larvae become fully fed within one hour. If not, the feed quantity should be adjusted accordingly, and the seedlings should be observed closely. The color of the water changes quickly after feeding, indicating good water quality.
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