First, all artificial nursery.
1. Selection of Pro-Shellfish
Choose healthy individuals aged 1 to 2 years for the pro-shellfish. These should have intact shells, be double-shelled, with tightly closed shells and sharp edges. The shell should be firm, and the soft tissue should be plump and well-developed. This ensures the best genetic quality for the breeding process.
2. Artificial Spawning and Insemination
Currently, fertilized eggs are mainly obtained through artificial spawning and insemination. The ratio of male to female pro-shellfish is usually (50–100):1. Mature females show a slightly yellowish color in their gonads, while males appear milky white and can be easily identified by the naked eye.
The process involves collecting eggs from the female into a bucket containing about 180 liters of seawater. Once a sufficient concentration of eggs is reached, an appropriate amount of sperm is added, followed by thorough mixing. Any foam that forms on the surface is removed. The mixture is left undisturbed for 10 minutes before being filtered through a 200-mesh sieve and transferred to the hatching pond.
The hatching density is typically around 150–250 kg per 10 tons of water, with the egg density controlled at 30 eggs per milliliter.
3. Larval Rearing
After approximately one day (at a water temperature of around 23°C), the fertilized eggs develop into "D"-shaped larvae. These larvae are then selected and transferred to different rearing pools for further development.
4. Feeding Cultures
Pacific oyster larvae are fed with various microalgae species, including *Phaeodactylum tricornutum*, *Nitzschia clostridium*, *Chaetoceros*, *Isochrysis galbana*, *Platymonas*, and cloud microalgae. Currently, cloud microalgae, *Nisslia* spp., and diatoms such as *Nitzschia closterium* and *Chaetoceros* sp. are commonly used. These algae are generally cultivated directly from seawater and are collectively referred to as diatoms in the marine environment.
(1) Cloud Microalgae Cultivation
Cloud microalgae is a type of green algae with small cells, measuring 1.6–2.6 microns in size. It has strong adaptability and is easy to cultivate, growing and reproducing year-round. It serves as the primary food source for Pacific oyster larvae. The initial inoculation density is 3.5–4 million cells per milliliter. As the population 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 about 4 million cells per milliliter. Since the reproduction rate is fast, a small amount of water is added at noon to keep the density below 7 million cells per milliliter. Fertilization is applied every other day using total nitrogen (N) at 5 g/m³, phosphorus (P) at 0.5 g/m³, and iron (Fe) at 0.025 g/m³.
(2) Seawater Diatom Cultivation
Diatoms are collected directly from seawater using a 300-mesh sieve. In Luoyuan Bay, where the water temperature is below 25°C, the dominant species is *Nitzschia closterium* (commonly called small diatoms). When the temperature exceeds 25°C, *Chaetoceros* becomes more prevalent. The initial inoculation density for seawater diatoms is 152,000–200,000 cells per milliliter. Due to their rapid growth, continuous water exchange is necessary to maintain a density below 400,000 cells per milliliter. Water is replenished every other day to ensure continuous culture. The daily fertilizer application includes nitrogen (3 g/m³), phosphorus (0.3 g/m³), iron (0.015 g/m³), and silicon (0.3 g/m³).
5. Larval Density Control
For "D"-shaped larvae, the ideal density is 4–5 per milliliter. At the early stage, the density should be around 2–3 per milliliter, and during the eye-stage larvae, it should be reduced to 1 per milliliter.
6. Feeding Practices
Currently, larvae are primarily fed with cloud microalgae and seawater diatoms. As the larvae grow, the feeding amount should be increased accordingly. After each feeding, the gastrointestinal fullness of the larvae should be observed. Most larvae should be fully fed within one hour. If not, the feed quantity should be adjusted, and the seedlings should be closely monitored. The color of the water changes rapidly after feeding, indicating good water quality and efficient digestion.
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