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Milk Breeding Artificial Propagation Technology
Milkfish, scientifically known as *Chanos chanos*, belongs to the order Milkweed (Oceanoptera), family Blindidae. In Taiwan, it is commonly referred to as "Milkfish," "Scaly larvae," "Anping fish," "National holy fish," "Harbour fish," and "Mackerel." This genus is represented by only one species in our country. Milkfish are euryhaline fish, known for their rapid growth, large size, and broad diet. They are widely farmed in Hong Kong and serve as a major aquaculture species in coastal areas of Southeast Asia. Since the 17th century, milkfish farming has been practiced in China, with significant development in Taiwan.
The breeding of milkfish is relatively simple, but one of the main challenges is the limited availability of fry. One of the key factors in successful milkfish cultivation is ensuring a steady supply of seedlings. Natural fishing of broodstock typically occurs in spring and autumn. In Hong Kong, fish rearing involves stocking in spring and harvesting in late autumn. Two primary methods are used to obtain mature broodstock: capturing sexually mature individuals directly from the sea or inducing maturity in immature fish through hormone treatment.
First, broodstock can be captured during non-breeding seasons and used for back-breeding. Fish weighing over 3 kg are ideal for this purpose. During the breeding season, broodstock with well-developed gonads are preferred. Milkfish have a lunar cycle that influences spawning, with more mature wild broodstock often caught one week before the full moon. In the Philippines, peak egg-laying occurs around the full moon. Gillnets, enclosures, and custom nets are commonly used for fishing broodstock. Gillnets are suitable for any water body, while seine nets are better for shallow or enclosed waters. Custom nets are effective in deeper open waters.
Second, transporting broodstock requires careful handling. Short-distance transport can be done using hand-cut nets or plastic bags filled with water. For long-distance transport, specialized barrels with controlled conditions are used. Broodstock aged 6 to 8 years are transported in 52-meter-wide tanks with a depth of 0.6 meters, containing an average weight of 2.16 kg per fish. These are packed in 20 barrels, with salinity maintained between 7 and 10, cooled with ice packs and oxygenated. The transport lasts 18 to 28 hours, with an 85% survival rate. However, fat eyelid injuries and cloudiness are common during transport. Mild cases usually recover, but severe ones can lead to blindness or death, which can be treated with antibiotics like nitrofurazone. Anesthetics or HCG administered at 555 IU/kg after transport help prevent gonad degeneration due to stress. Before oxytocin application, females should be acclimated gradually to a salinity of 35.
Third, feeding broodstock plays a crucial role in gonad maturation. The feed must contain 32% to 40% protein to promote reproductive development. Common feeds include gluten, wheat flour, soy flour, squid compound feed, shrimp granules, and mixed fish and crustaceans. In high-salinity ponds, Artemia is essential for sexual maturity. Feeding amounts range from 2% to 5% of the fish's body weight.
Fourth, environmental conditions are critical for broodstock maturation. The minimum water temperature for mature fish is 20°C, and spawning requires at least 24°C. Maturation can occur within a wide salinity range, from 8 to 39, with natural maturation at 39. Spawning requires standard seawater salinity. Light intensity can stimulate gonad maturation but is not a determining factor. Broodstock can be raised in earthen ponds, concrete pools, fiberglass tanks, or canvas pools with proper water circulation and aeration systems.
Fifth, induced spawning is necessary under artificial conditions. Female fish rarely spawn naturally, so hormone injections are required. Before spawning, oocyte diameter must exceed 660 microns. A single injection of 10,000 IU HCG/kg or 250 μg LHRH-A/kg is typically used. Ovulation is checked using an ovulation tube, and sperm maturation is observed by gently pressing the male abdomen. Artificial insemination is then performed when both gametes are mature.
Sperm preservation is also important. Milkfish serum is the best solution for cryopreservation, while dimethyl sulfoxide outperforms glycerol as an antifreeze agent. Frozen semen stored in liquid nitrogen shows higher fertilization rates and larval survival than fresh or cold-stored semen.
Egg characteristics include transparency, round shape, no oil globules, and a diameter of 1.2 mm. Fertilized eggs sink in salinity 30 but float in salinity above 34. They hatch in 25–28.5 hours at temperatures between 26.4°C and 29.9°C, with hatching rates exceeding 80%. Eggs cannot develop in salinity below 16, making it impossible for milkfish to breed in such conditions.