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Black fish seed breeding key technologies
When black mullet larvae hatch from their egg membranes, their development is still incomplete, making them very delicate and challenging to manage. The key issues observed during this stage include:
1. **Sensitivity to Environmental Factors**: Early-stage fry are easily disturbed by water currents and waves, often sticking to the gauze in the rearing container and dying as a result.
2. **Risk of Hypoxia**: On the second night after hatching, the fry tend to cluster together, which can lead to oxygen depletion in localized areas, resulting in hypoxic death.
3. **Vulnerability to Infections**: Without scales on their bodies, the fry are prone to injuries that can lead to infestations by protozoans such as *Trichodina*, *Carchesium*, and *Euglena*.
4. **Water Quality Deterioration**: Dead eggs and fry decompose, releasing harmful substances and oil droplets that pollute the water. To mitigate this, it's recommended to transfer the fry to a larger net cage or directly into a lower pool once they begin feeding normally.
**Feeding Strategy**:
The first food for newly hatched black mullet fry should be small zooplankton, such as rotifers and copepods, with individual sizes kept below 200 microns. These can be cultured in the fry pond beforehand. If zooplankton is scarce, a 100-mesh plankton net can be used to collect them, followed by filtration through a 40–60 mesh cloth to concentrate microzooplankton. A small amount of oxytetracycline can be added to the filtrate, and after 30 minutes, the mixture can be fed to the fry. Feeding should occur six times daily, every four hours, following the principle of "small portions, frequent meals." As the fry grow and become more active, the feeding amount can gradually increase while the number of feedings decreases, eventually settling at two meals per day.
**Preventing Clustering**:
Especially within the first seven days after hatching, the fry tend to cluster, which can lead to local hypoxia and high mortality. To prevent this, it’s important to gently stir the water using tools like a fishing net, helping to disperse the fry and improve survival rates.
**Maintaining Water Quality**:
Regular removal of dead eggs and fry is crucial to avoid decomposition and the release of harmful substances. Net cages should be cleaned regularly, and cement pools should be siphoned and drained. Ideal conditions involve using microfiltered water. For earthen ponds, water changes should be done frequently, gradually increasing the volume. The temperature difference between new and existing water should not exceed 2°C, and initial water exchange should be around 60–70%, reducing to 30–50% after two weeks. Water changes should be adjusted based on water quality.
**Disease Prevention**:
Newly hatched fry are susceptible to fungal infections and protozoan infestations. After the fry have fully emerged from the membrane, any remaining egg membranes should be removed, and the fry should be treated with 1–2 doses of 15 ppm formalin. Freshwater used for exchange should be pre-treated with 15–25 mg/L of quicklime or a chlorine-based disinfectant in a holding tank to reduce contamination risks.
**Stocking Management**:
After about three weeks of culture, the fry will reach 2.5–3 cm in length. At this stage, size differences can lead to cannibalism and increased disease risk. To prevent this, periodic sorting and gradual reduction in stocking density should be performed, allowing smaller fish to be moved to separate breeding ponds. This helps maintain healthy growth and reduces stress among the population.