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Artificial reproduction of cobia
Cobia, also known as sea trout, is a tropical fish found in the Atlantic, Indian, and Pacific Oceans, excluding the eastern Pacific. Major fishing regions include Pakistan, the Philippines, and Mexico, while China's coastal areas also host this species, albeit with lower production. As a carnivorous fish, cobia has flavorful meat that is ideal for raw fish fillets. It adapts well to captivity and can be fed artificial feed, growing rapidly—reaching 6–8 kg annually. Due to its fast growth and high market value, it holds great potential for marine cage farming.
**Biological Characteristics**
First, diet: In the wild, smaller cobia primarily eat shrimp, crabs, and cephalopods, making up about 80% of their diet, followed by small fish. Once they grow over one meter, fish becomes the main food source, accounting for 80% of their intake. Under captive conditions, cobia can be fully domesticated and fed pelleted feed.
Second, temperature: Cobia is a warm-water species and cannot tolerate low temperatures. The optimal temperature for egg development is between 24°C and 31°C. For juvenile fish (10g–15g), water temperatures below 20°C significantly reduce feeding, and at 19°C or lower, they stop eating. At 16°C, they begin to die. Between 22°C and 34°C, they show active feeding, but at 36°C, even though they still eat, mortality begins to rise.
Third, salinity: Cobia is a euryhaline species, thriving in salinities between 4‰ and 35‰. When salinity increases beyond 35‰, feeding declines. At 40‰, feeding is halved; at 43‰, only weak feeding occurs; and at 47‰, death begins. If salinity drops from 30‰ to 5‰, survival is possible, but at 3‰, feeding stops, and death follows. Prolonged exposure to extreme salinity levels can slow growth or weaken immunity. Larger cobia are less tolerant of low salinity, and when salinity falls below 8‰, they stop feeding.
Fourth, oxygen consumption: Fish fry weighing 0.5g consume around 1.08 mg O₂/g/h at 30°C, with lethal dissolved oxygen levels at 1.7 mg/L. At 28°C, oxygen consumption drops to 0.86 mg O₂/g/h, with a lethal level of 1.5 mg/L. Higher temperatures increase both oxygen demand and tolerance limits. Larger fish require more oxygen.
Fifth, reproductive habits: During breeding season, female cobia develop distinct black and white stripes and have a swollen abdomen, while males lose these markings and have a smaller belly. Maturity varies by region—Zhanjiang cobia reach sexual maturity at 2 years old, with females over 8 kg and males over 7 kg. A female weighing 8 kg produces about 1.28 million eggs, each measuring 125–137 µm. Spawning typically occurs between April and October in the Gulf of Mexico, and between February and May in southern Taiwan, with optimal temperatures between 24°C and 29°C.
**Artificial Propagation**
Since 2000, Zhanjiang Ocean University has successfully bred cobia in captivity. Initial efforts produced 70,000–8 cm fry, with 2,000 selected as broodstock. From November 2001 to April 2002, vitamins and minerals were added to their feed to enhance gonad development. By 2002, approximately 360,000 7 cm fry were raised.
Artificial spawning involves injecting broodstock with hormones like LRH-A2, HCG, and DOM. Some fish were induced naturally through environmental stimulation. Fertilized eggs are slightly yellow, oval, and floating, with diameters of 135–141 µm. They hatch within 24–30 hours depending on water temperature.
Newly hatched larvae are about 3.5 mm long, with melanin distribution and oil globules beneath the yolk sac. They swim suspended in water, feeding after 60–66 hours. Their diet progresses from rotifers to copepods, Artemia, and finally minced fish. Nutrient-enhanced feeds, such as chlorella-enriched rotifers and cod liver oil, improve survival rates.
Cultivation in earthen ponds follows similar methods to freshwater fish. Clearing and fertilizing the pond ensures sufficient food organisms. Feeding must coincide with peak food availability to avoid competition. Regular size grading and transfers to seawater cages help achieve marketable sizes of over 7 cm.