The red sea-bream Pagrus major, also known as the red porgy, is a very valuable species in Japan, because it is considered a symbol of good fortune and eaten on all auspicious occasions. Commercial catches of the species are reported to be declining, and so its propagation is under-taken not only for production of market-sized fish in captivity, but also for stocking open waters for enhancing natural populations. As a result of several years of scientific effort, it is now possible to induce spawning and artificially propagate the species.
Mature brood fish can be obtained from commercial catches or from captive stocks. Thesexes are easily distinguished, especially during the spawning season. The male has darker coloration and a more angular head. Maturation can take place in net cages or concrete tanks. The spawning season extends from April to June, with a peak in early May. Three- to four-year-old fish are used for breeding. Females of that age, which weigh about 1kg, spawn approximately 300000 eggs. According to Kittaka (1977), the average number of eggs spawned per female has been recorded as 4800000 for 6–13-year-old fish and 2700000 for three-to-four-year-old fish. Fully mature females and males held in tanks with frequent replacements of sea water spawn naturally in temperatures between 15 and 22°C. Spawning continues for several days and the eggs are collected with siphons or through overflow arrangements. An alternative is to strip the mature brood fish and fertilize them artificially.
A mature egg measures about 1.2mm in diameter and has a sticky covering, which will slowly dissolve in sea water. Fertilized eggs are highly susceptible to changes in temperature and high-intensity light. Hatching is performed in 50–100 ton capacity tanks with circulation of sea water. The stocking rate of eggs in the tanks is generally 30000–40000 per ton. The eggs will not hatch at temperatures below 10°C. The black pigment in the large globule (0.25mm in diameter) in the eggs apparently permits light penetration of roughly 100–3000lux, and so care has to be taken to ensure that the illumination in the hatching tanks does not exceed the maximum of 3000lux. The eggs are normally pelagic, but if the specific gravity of the water is below 1.023 (at 15°C) they may sink to the bottom, in which case their development will be affected and the hatching rate poor. So it is necessary to have an adequate supply of water of the required specific gravity, corresponding to a salinity of 33.5ppt.
Under favorable conditions, the eggs hatch out in about 60 hours at a temperature of 15°C or in 40 hours at a temperature of 18°C. The larvae become active on the second day, and the yolk sacs are absorbed by the third day. They are then transferred to rearing tanks and fed on live food. The most common means of feeding larvae is by the addition of the so-called green water. Green sea water is produced in outdoor
tanks filled with clean sea water and fertilized with a chemical fertilizer at the rate of 500g/ton water. Phytoplankton is inoculated to stimulate planktonic growth and within 10 days the tanks develop a rich green growth of phyto-and zoo-plankton. It is reported that such green water can sustain a population of about 40000 larvae/ ton water. After a few days, the water in the tank is freshened gradually with fresh sea water. At this time the larvae are fed with live food such as oyster larvae, rotifers, etc., collected from outside sources. Ten-day old larvae are quite active and can be fed on brine shrimp (Artemia) nauplii. Like the eggs, the larvae also require suitable light, but it appears that the light requirement varies according to the individual and the stage of growth. Because of this, it has been recommended that the rearing tank should have zones with different light intensities (for example, 0lux to 2000lux) so that the larvae can select the preferred light intensity at any particular time.
By about the twentieth day, and at a length of about 10mm, the fry show signs of benthic life. At this stage they consume small poly-chaetes and minced shrimp meat. Fry of 20mm size are fed on minced white fish and shrimps. Grow-out facilities are usually stocked with fry of this size.
The red sea-bream is generally grown in floating net cages. The stocking density is about 6– 8kg/m3 water and in about 12–18 months they grow to market size. Most farmers feed them with frozen fish such as anchovies, sand eels, etc. Pelleted diets are now commercially available, but many farmers seem to prefer to use mash diets, or minced fish mixed with a formulated dry powder diet.
As mentioned earlier, the fry and fingerlings are also released into the sea for enhancing natural populations. For this purpose, fry are acclimatized and grown in floating cages for a few weeks until they attain a size of about 5–
7cm. Then they are released in suitable areas, where protective devices like concrete blocks or plastic strips are placed to serve as shelters for the fry.
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