Cobia is also known as the sea clam, which belongs to the genus Cichlid, Coelidae, and Cobia. It is distributed in tropical waters such as the Atlantic Ocean, Indian Ocean and the Pacific Ocean (excluding the East Pacific Ocean). The individual can reach up to 1.5 meters and weigh 43 kg. The world’s major fishing producing countries are Pakistan, the Philippines, and Mexico. China’s coastal areas are also distributed, but their production is low. According to years of research by researchers, cobia is easily domesticated and eats artificial feed. Its growth rate is fast, and its annual weight gain can reach 6 kg to 8 kg. It is one of the fastest-growing and most industrialized fishes in marine cage culture. It is comparable to Norway's main farmed fish, the Atlantic Ocean.
First, biological characteristics
1. External morphology
Cobia fish body extended, nearly cylindrical, slightly sideways. Head flat, wider than tall. Eyes are smaller, located on both sides of the head, and the upper edge of the head is a few and there is no fat film. The mouth is large, the front position, and the jaw is slightly longer than the upper jaw. The upper and lower jaws, the vomer, the sacrum, and the tongue surface all have villous tooth bands. The front humeral edge is smooth. The body, cheeks, upper edge of the lid, top of the head, and fins are all covered with small round scales. The lateral lines are slightly corrugated. Short and thick. The pylorus is small in size and numerous in size. Innocent. The back of the body is dark brown and the belly is greyish white. Its fins are black, but the pelvic fins and the upper and lower edges of the caudal fins are white.
2. Food habit
In the natural sea area, smaller cobia mainly feed on shrimps, crabs, and cephalopods. Shrimp, crab, and cephalopods account for about 80% of the total food, followed by fish. For cobia that are more than 1 meter in length, fish is mainly eaten, and fish accounts for 80% of its total food. Cobia can be artificially fed in artificial pellets under artificial breeding conditions.
3. Suitable temperature
Juncao is a tropical sea fish that is not tolerant of low temperatures. After testing, the suitable water temperature for embryonic development is between 24°C and 31°C; at the larval stage (body weight between 10g and 15g), the water temperature is as low as 20°C to 21°C, the food consumption is significantly reduced, and no feeding at 19°C, 17°C～18 °C activity weakened, remained at the bottom of the water, and began to die at 16°C. The water temperature was 22°C to 34°C. There was significant feeding activity. When the water temperature rose to 36°C, it started to die despite feeding.
4. Salt salability
For salty fishes, the salinity is 8‰ to 35‰. Larger cobia have lower endurance to low salinity, with salinity less than 8 phr, ie no feeding activity. For food fish farming, seawater salinity should be maintained at 10 ‰ or more.
5. Oxygen consumption
After testing fish fry with an average body weight of 0.5 g, when the water temperature is 30°C, the oxygen consumption is 1.08 mg/ghr, and the lethal dissolved oxygen amount is 1.7 mg/l; when the water temperature is 28°C, the oxygen consumption is 0.86 mg/ghr, causing death. The dissolved oxygen was 1.5 mg/l. Under normal circumstances, when the water temperature is higher, the oxygen consumption is relatively increased; the larger the individual, the higher the oxygen consumption.
6. Genital habits
(1) The sex and sexual maturity of the smallest body type
During the reproductive season, the black and white stripes on the back of the cobia females become more pronounced and the abdomen protrudes, whereas the stripes of mature males are not obvious or disappear and the abdomen is small. However, judging from the body color is not always accurate, but it is still necessary to supplement the body shape for judgment. In cage-raised cobia, sexual maturity varies from place to place. The Zhanjiang area is 2 years old. The male body weight is more than 7 kilograms and the female body weight is more than 8 kilograms. The relative amount of eggs is 1 kg body weight 160,000 capsules, the smaller the egg size.
(2) Spawning season
In the natural sea area, cobia are many spawning fish and have a long reproductive period. In the northern Gulf of Mexico off the east coast of the United States, mature broodstock can be found from April to October. In southern Taiwan, the peak of spawning occurred from the end of February to May, and spawning was sparse until October. Suitable spawning temperature is 24°C to 29°C.
Second, artificial breeding
Artificial labor
The broodstock with its large abdomen and exposed gonad outline should be selected for oxytocin production in darker oxytocin pools. Oxygen production uses luteinizing hormone-releasing hormone analogues (LRH-A2), human chorionic gonadotropin (HCG), and domoxone (DOM), two injections in the pectoral fin base part of the broodstock, which accounts for 1/1 of the total 3, the second time accounts for 2/3 of the total. Some broodstocks with loose belly and obvious gonads are put into urine production ponds, and eco-stimulation methods are used to promote their natural spawning.
2. Hatching
Cobia fish eggs are round and slightly yellow, floating eggs, fertilized eggs, slightly swelling of the egg membrane, egg diameter 135 microns to 141 microns. Poor eggs are opaque and poorly buoyant and should be removed. The number of eggs per kilogram is about 500,000. When the water temperature is between 24°C and 26°C, the membrane can be hatched within 30 hours; when the water temperature is 28°C to 30°C, the membrane hatches within about 24 hours.
3. larval rearing
Compared with other marine fish, newly hatched larvae are quite large in size. The total length is about 3.5 mm. The body has melanin distribution. The oil globules are below the yolk sac and the fins are translucent. The larvae are suspended in water at rest. Facing upwards, it is horizontal or inclined. At 12 hours after membrane rupture, the larvae were 4 mm in length and about 4.8 mm in length in 24 hours. The larvae were dark brown in color, with a silvery yellow color beginning from the back of the head to the tail. Occasionally, the larvae rushed for a short and anxious swim. , mainly suspended in water. The third day old larvae had a total length of 6 mm, a mouth width of about 0.4 mm, a large mouth, brown pigmentation around the spine, a dark brown trunk, and a slightly silvery tail. From day 3, that is, 60 to 66 hours after membrane rupture, the larvae started feeding.
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