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Milk Breeding Artificial Propagation Technology
The species of the Milkweed order, specifically from the family Blindidae, is commonly known as the Milkfish. In Taiwan, it is referred to by several names, including "Milkfish," "Scaly larvae," "Anping fish," "National holy fish," "Harbour fish," and "Mackerel." This genus contains only one species in our country. The Milkfish is a euryhaline fish, meaning it can tolerate a wide range of salinities, and it grows quickly with a broad diet. It is widely farmed in Hong Kong and is a major aquaculture species in coastal areas of Southeast Asia. Since the 17th century, fish farming in Taiwan has been well established, with large-scale development taking place over time.
Milkfish farming requires relatively simple conditions, but there is a challenge in securing sufficient seedstock. One of the key factors in successful breeding is ensuring an adequate supply of fry. Natural fishing usually occurs in spring and autumn, while in Hong Kong, fish are stocked in spring and harvested in late autumn. There are two primary methods for obtaining mature broodstock: capturing sexually mature fish directly from the wild or using hormone-induced maturation in immature individuals.
When catching broodstock outside the breeding season, fish weighing more than 3 kg are preferred for back-breeding. During the breeding season, those with better gonadal maturity are selected. Milkfish have a monthly reproductive cycle, with wild broodstock often captured one week before the full moon. In the Philippines, spawning peaks around the full moon. Gillnets, enclosures, and custom nets are used for fishing broodstock. Gillnets are suitable for any water area, seine nets work best in shallow or enclosed waters, and custom nets are ideal for deeper open waters.
Transportation of broodstock depends on distance. Short-distance transport can be done using hand-cut nets or plastic bags filled with water, while long-distance transport uses specially designed barrels that allow for manual control. For example, 6- to 8-year-old broodstock are transported in 52-meter-wide tanks with a depth of 0.6 meters, each holding 20 fish with an average weight of 2.16 kg. The salinity is maintained at 7–10, and ice packs are used to cool the water. Oxygen is supplied, and the journey lasts 18–28 hours, resulting in an 85% survival rate. However, during transport, milkfish often suffer from eye injuries and cloudiness, which may lead to blindness or death if severe. Antibiotics like nitrofurazone can help treat these issues. Administering anesthetic agents or HCG (555 IU/kg) post-transport can prevent gonad degeneration due to stress. Before oxytocin application, females should be gradually acclimated to a salinity of 35 through a 1–4 gradient.
To promote gonad maturation, broodstock must be fed a diet containing 32–40% protein. Suitable 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. Fish are typically fed 2–5% of their body weight daily.
Water temperature plays a crucial role in maturation. Sexual maturity begins at 20°C, and spawning occurs at 24°C. Using LHRH-A implants, maturation can be achieved at 20–22°C. Salinity also affects maturation, with a minimum of 8 and natural maturation at 39. Spawning only occurs at standard seawater salinity or higher, and sudden changes in salinity can inhibit reproduction. Milkfish can spawn in inland salt lakes with salinity up to 13.
Light intensity stimulates gonad maturation but is not the sole determinant of spawning. Broodstock can be reared in earthen ponds, concrete pools, or fiberglass tanks with controlled water flow and aeration systems. Stocking density varies between 20–40 cubic meters per fish, with a female-to-male ratio of 1:1 to 1:2.
Artificial induction is often necessary for maturation, especially under farming conditions. Hormones like LHRH-A and 17-methyl testosterone are commonly used. Light cycles are also adjusted—6 hours of light, 18 hours dark; 12 hours light, 12 hours dark; and 18 hours light, 6 hours dark—to induce maturation.
Induced spawning involves hormone injections, such as 10,000 IU HCG or 250 μg LHRH-A per kg of body weight. Ovulation is checked via ovulation tubes, and sperm release is observed by gently pressing the male’s abdomen. Artificial insemination is then performed when both gametes are mature.
Sperm preservation is most effective using milkfish serum as a cryopreservation medium, with dimethyl sulfoxide outperforming glycerol. Frozen semen stored in liquid nitrogen shows higher fertilization rates and larval survival compared to fresh or chilled samples.
Eggs are transparent, round, and yellow, measuring about 1.2 mm in diameter. They sink in seawater with salinity 30, float above 34, and hatch after 25–28.5 hours at 26.4–29.9°C. Hatching rates exceed 80%. Eggs cannot develop in salinity below 5, and while they can be fertilized at salinity above 16, hatching fails. Thus, milkfish cannot breed in salinity below 16.