(1) Artificial seedlings thrive best under specific environmental conditions. The optimal salinity should be maintained between 32 and 35 parts per thousand, with a temperature range of 19 to 24°C. Dissolved oxygen levels should be around 0.0005%, and the stocking density should fall between 50 to 250 individuals per liter. Slightly larger aquaculture systems can improve survival rates at lower densities, but this requires more feed. To prevent light reflection and direct sunlight, cement tanks or non-reflective black polyethylene containers are commonly used.
The rearing tanks should be prepared a few days before hatching, and the water stored in them must be filled. Seawater needs to be sterilized and filtered before use. It should maintain a salinity of 32–35‰ and a temperature of 20–22°C before introducing the fry. Prior to adding fish, the tanks should be inoculated with green algae or other phytoplankton species to provide food for rotifers. Green algae is typically the preferred choice for this purpose.
After approximately 60 hours, the eggs hatch completely, and the newly hatched fry are transferred to the culture pool using a siphon tube. Before siphoning, the circulation of water should be stopped. At this point, the fry float on the surface, while the egg membranes and unhatched eggs sink. Care must be taken during the process to avoid transferring these unwanted materials along with the larvae.
The newly hatched fry measure about 2.2–3.5 mm in length. They have oil globules in the posterior part of the yolk sac and possess 24 sarcomeres. Their bodies are bent over the yolk sac, and they begin feeding 3–5 days after hatching. The diet for larvae includes oyster eggs, diatoms, green algae, copepods, and Artemia. Different types of food are required at various developmental stages (see table below for details).
Days post-hatch | Type of Bait
3–13 | Oyster eggs (fertilized)
5–18 | Rotifers
10–14 | Copepods
16–40 | Artemia nauplii
29–41 | Cooked protein, rice bran, flour, etc.
When artificial seedlings are cultured, both water and air temperatures should be kept at 20–22°C, with a salinity of 32‰–35‰. The supply water is usually provided by a water tower and is sterilized through filtration and ultraviolet treatment. On day four after hatching, the water in the rearing tank should be diluted with fresh water, reducing the salinity by 0.5‰ daily. By day seven, the salinity should reach 30‰, and by day 30, it should gradually decrease to 20‰. Water quality parameters such as temperature, salinity, dissolved oxygen, pH, nitrate, ammonia nitrogen, sulfide, and phosphate should be monitored daily. The fry can tolerate: 3.00 µg/L of nitrite, 90 µg/L of nitrate, 30 µg/L of ammonia, 9 µg/L of phosphate, and 5 µg/L of hydrogen sulfide. Dissolved oxygen must be above 0.0007% on day one, with a pH of 7.9–8.3. The rearing tanks should be cleaned daily using a small siphon to remove dead seedlings and leftover feed. Recirculating systems help with self-purification, and debris can be removed via siphoning or from the outlet, especially when the water exchange rate is high. Between 30 and 50 days, the fry grow rapidly and can be fed with wet synthetic feeds, older Artemia, and large copepods.
For the first 40 days, do not move or catch the eel fry, as this can lead to high mortality. Moving should only be done after 50 days of hatching.
(b) Fish seedlings, such as carp, are often captured from the sea and temporarily conserved before being domesticated. Alternatively, artificial seedlings can be moved to fingerling ponds after being raised for over 50 days. Fingerling ponds should cover an area of 0.07 to 0.15 hectares, with a water depth of 1.5 to 2 meters. The pond bottom should not contain excessive silt. The water should maintain a certain level of salinity. Before stocking, the pond should be thoroughly cleaned.
Each hectare can hold 45,000 to 60,000 fry of 3 cm in size. After placing the fry in the pond, a mixture of barley and bean cake is fed twice daily, either as dry powder or mixed into the water, which helps support the growth of carp seedlings. Once the fry are all in the pond, fertilization begins to cultivate zooplankton, with the fertilizer amount gradually increased. After about a month of feeding, the eel fry can grow to 5–7 cm and then be transferred to adult ponds for further cultivation.
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