The gilthead sea-bream (S. aurata) is a highly priced species in the Mediterranean and neigh-bouring countries, and because of diminishing catches from open waters there is considerable interest in its intensive culture. The traditional polyculture of the species in coastal impoundments or vallis is based on wild fry, as it does not breed naturally in confined waters. Much of recent research on the species has been directed towards developing a suitable system of artificial propagation to produce fry and fingerlings.
The gilthead sea-bream are characterized by protandric hermaphroditism, which occurs from the second to the fourth year of their life. The natural breeding season in the Mediter-ranean region is between October and December, when the water temperature varies from 13 to 17°C.
Maturation of captive fish can be induced by the infection of HCG, the effective dose varying with the state of gonadal maturity. Spawning has been successfully induced with a single injection, but more frequently with a series of two to nine injections of 800–2000IU/kg body weight.
The brood fish are kept in salinities between 35 and 37ppt at temperatures of 17–21°C. The injections may be given at intervals of two to three days, and spawning occurs four to five days after the first treatment. If spontaneous spawning does not occur, stripping and artificial fertilization are carried out. The eggs measure 0.9–1.1mm in diameter and the estimated annual fecundity is around 500000/kg. It is reported that mature gilthead sea-bream can also spawn naturally without any hormone treatment in tanks with a constant exchange of sea water of the required temperature (as observed in sea-bass). Incubation of the eggs can be done in any standard incubator or even in 200–600l tanks with circulating sea water of about 36ppt salinity and partial aeration. At temperatures around 15–20°C, hatching takes place in about 50 hours. The hatchlings are about 2.5–3mm in length and start feeding from the third or fourth day, depending on the water temperature.
Different types and sizes of tanks have been used for larval rearing. Indoor tanks of about 200–600l capacity with circulating sea water of salinity in the range 26–37ppt, artificial illumination (600–3500lux) for 12–16 hours a day and aeration have given satisfactory results. The recommended larval density in the tanks is 10 per l. The first food of sea-bream larvae is live organisms (as in the case of sea-bass larvae). Proper larval feeding is important not only in promoting good survival rates, but also in preventing deformities in fry. A suitable sequence can be 20–25 Brachionus per ml from the fourth day, 8–10 Artemia nauplii per ml from the sixteenth to the fortieth day and 5–8 Artemia metanauplii and juveniles per ml from the fortieth day to the fry stage. Rotifers and cope-pods are also used as early food and chopped mussels or finely minced fish are given from about the fifty-fifth day. About 90–100-day-old fry are used for on-growing. However, the maximum survival rate reported is only about 16 per cent.
Information available on the results of mono-culture of gilthead sea-bream is limited. Experimental work carried out in tanks shows that they can be raised to marketable size of 200–400g on artificial feeds (trout pellets) in one to two years. Growth rates in floating cages are similar to those in ponds. A pilot farm in Turkey is reported to have grown 5g juveniles to a weight of over 250g in 12–18 months in cages, fed on commercial pellets and trash fish. Fingerlings of about 80g, when stocked in cages at a low density of 36–78 per m3, are reported to have grown to an average weight of 300g in six to seven months, in Israel. At a density of 180/m3 the fish reached an average weight of 315g at an age of 15–16 months, when fed on a high-protein pelleted feed. Nutritional studies have shown that feed with a 40 per cent protein level gives optimal utilization. Pellets of 10mm diameter have been found to be efficient in growing fish of 100g and above.
Fresh sea-bream fetches the best price and so harvests from ponds or cages are, as far as pos-sible, sold fresh on ice.
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