Production of seedlings
Although methods of artificial propagation of yellowtail have been developed, the farmers depend largely on seedlings collected from natural sources.
Because of the increasing number of seedlings required by the farming industry and the possible adverse effects of the removal of large numbers of larvae, the government enforces restrictions on the quantity that can be caught every season. Yellowtail are migratory fish which move into the offshore waters from March to May, where they spawn. The larvae are brought towards the shore by the Kuroshio current along with floating sea-weeds. It is from these coastal areas that they are caught with large encircling nets fishing boats, from about May to June. The boats carry storage tanks in which the larvae are kept until the boats return to port. The larvae measure25–40mm in length.
Aritificial propagation of yellowtail has been carried out with mature fish caught in drift nets from the sea. Eggs stripped from mature females have been fertilized with milt from captured males. Experimental work on the use of hormones for maturing brood fish and inducing spawning has also been performed. Maturation can be induced by the injection of Synahorin at the rate of 4IU/kg of fish. Fertilized eggs hatch out in about 51–68 hours in water temperatures ranging from 18 to 24°C. As starter feed, the hatchlings are given eggs of sea urchins or oysters. Oyster larvae, enriched Artemia and Brachionus form the main food when they startmore active feeding, and later they take to copepods. The fry can be fed on minced juvenile fish and fish meal.They grow around 10mm in about 20 days, 17mm in a month and about 63mm in two months. A month-old fry, which will have developed the characteristic stripes on the sides, becomes cannibalistic.
Because of the cannibalistic habits of the fry, the seedlings collected from natural sources or propagated artificially are carefully sorted out according to size for rearing. The sorted fry can be reared separately in small floating, fine-mesh net-pens. The pens are generally 2–50m2 in area and 1–3m deep. The fry are fed on minced fish (such as sand eel and horse mackerel) and shrimps. A high fat content in the feed and feeding above 80 per cent satiation are avoided. Best results have been obtained by feeding crustaceans or white-meat fish. Shrimp flour made into a paste forms an excellent food for yellowtail fry. Some farmers feed the fry with zooplankton, and lights are often hung above the cages to attract the zooplanktonic organisms. The fry rearing usually takes a period of four to six weeks, during which time they grow to approximately 8–10cm in length and 25–50g in weight.
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