Usually every hatchery has its own broodstock unit, where breeders of various age groups, from 1 year-old males to 5-year old females, are kept under long-term stocking conditions. Breeders can come either from a farm or from the wild.

At the beginning of the spawning season selected batches of breeders are transferred from their long-term location to the spawning tanks. The control of the sex ratio in spawning tanks is a very important factor for gilthead seabream and precautions need to be taken because sex reversal is socially determined. The presence of young males at the end of the spawning period, for instance, increases the number of older fish that become females. On the other hand, the occurrence of older females reduces sex reversal in younger fish.

Out-of-season spawning

Gilthead seabream broodstock may be conditioned by environmental manipulation in order to extend or modify reproduction time. Fish are stocked in tanks equipped with a water heating/cooling system and computerized control of temperature and light intensity. Sexual maturation is obtained by exposing the broodstock to photoperiod and water temperature conditions that occur during the natural spawning period. Female spawning can be obtained by GnRHa(1) (D-Ala6; Pro9Net-mGnRH ) inoculation (5-20 mg/kg).

There are two principal systems of gilthead seabream larval rearing called small-scale and large-scale. The small-scale (<10 m³) rearing system is characterized by maximum control of environmental parameters and is conceived in order to produce a large number of juveniles (150-250/litre). The large-scale (~200 m³) technique simulates a natural ecosystem. This technique guarantees much better larval quality than the small-scale system, but produces far less juveniles (maximum 10/litre).

Gilthead seabream larvae generally deplete their yolk sacs after 3-4 days of endogenous feeding. At this stage, the eyes are pigmented and the mouth developed, allowing the larvae to prey on larval organisms. In most rearing systems the first living organisms used for larval feeding are rotifers (e.g. Brachionus plicatilis); these are chosen due to the relative ease with which they can be culture on a large scale. After 10-11 days, rotifers are integrated with Artemia salina nauplii until the larvae accomplish metamorphosis (32-35 days post-hatching). Prior to being fed to the larvae, both rotifers and artemia are routinely enriched with commercial lipid preparations, to enhance their levels of certain essential fatty acids (EPA; DHA) and vitamins that are critically important for good growth, development and survival. In Mediterranean hatcheries microalgae (e.g. Chlorella sp., Isochrysis galbana, Pavlova lutheri, Nannochloropsis oculata, N. gaditana, Dunaliella tertiolecta) are used both for rotifer production and to improve the water quality in the larval tanks, creating the so-called 'green water' that is used during the initial rearing phases.

Weaning with a dry high-protein (50-60 percent) formulated diet takes place when fish reach a weight of 5-10 mg.

Juveniles at about 45 days old are generally moved into a dedicated section of the hatchery equipped with larger round or rectangular tanks (10-25 m³), where weaning takes place. The weaning stage is a truly intensive rearing system. Initial fry density is generally 10-20/litre at a temperature of 18 °C and salinity of 35-37‰. Final density can reach 20 kg/m³ of 2-3 g fish. Feed is presented at 2-hour intervals from 08.00 to 20.00, using increasing percentages of artificial feeds composed of 150-300 µm particles. Dry feed should initially be presented at about 20 g/m³.

Gilthead seabream can be farmed in various ways: in coastal ponds and lagoons, with extensive and semi-intensive methods; or in land-based installations and in sea cages, with intensive farming systems. These methods are very different, especially regarding fish farming density and food supply.

Extensive system

This system is based on the natural migration of euryhaline fish, when the fish may be caught, generally in typical fishing traps. Since this practice provides a very limited and unpredictable source of natural juveniles, many modern commercial extensive production units rely on both wild-caught and hatchery-reared juveniles. Generally, 2-3 g gilthead seabream are seeded into the lagoons in April-May.

Under these systems gilthead seabream reach the first commercial size (350 g) in 20 months and are usually farmed together with mullets, eels and European seabass. In North Mediterranean lagoons, wintering in deep basins, with freshwater/seawater stratification, is needed in order to preserve one year old gilthead seabreams.

The total production of this kind of polyculture ranges from 30-150 kg/ha/yr according to lagoon productivity. In north-eastern Italian lagoons the production of gilthead seabream represents 15-30 kg/ha/yr of the total. During the production cycle the fish feed on natural lagoon resources; no supplementary food is provided. In extensive fish farming the fish density generally does not exceed 0.0025 kg/m³.

Semi-intensive systems

In this system human control of the farming environment is greater than in the extensive system. It may simply involve seeding lagoons with juveniles pre-fattened in an intensive system, to minimize mortality and shorten farming time. In this case it is also possible to fertilize the farming area in order to increase natural food availability. Other types of semi-intensive farming involve more control, and comprise the provision of artificial feed and supplemental oxygen. This type of semi-intensive farming system is usually carried out in net enclosures within limited areas of the lagoons. The final production can vary widely, according to the size of the juveniles stocked and the amount of feed presented. The density in semi-intensive systems does not normally exceed 1 kg/m³ and production ranges between 500-2 400 kg/ha/yr.

Intensive systems

Intensive grow-out normally follows other intensive farming phases, namely reproduction, larval rearing and pre-fattening, as described above. Gilthead seabream intensive pre-fattening and grow-out phases may be carried out in land-based installations with rectangular concrete tanks that vary in size (200-3 000 m³) according to fish size and the demands of production. Grow-out may also occur in sea cages, either in sheltered or semi-exposed sites (floating cages) or totally exposed sites (semi-submersible or submersible cages).

Intensive systems may be stocked with juveniles purchased from separate hatcheries, but large production units normally rear their own. In intensive grow-out systems the FCR is usually very favourable (about 1.3:1).

When gilthead seabream are reared in tanks very high densities are used, ranging from 15-45 kg/m³ and massive oxygen injection is needed to ensure fish survival. Under excellent conditions (18-26 °C), pre-fattened small gilthead seabreams (5 g) reach first commercial size (350-400 g) in about one year.

Ongrowing in sea cages is simple and economical; it is the fattening system normally used in the Mediterranean basin. Although densities (10-15 kg/m³) are lower than in tanks, there are great advantages that make cages farming more profitable. For example, there are no energy costs for pumping, aeration, or post-rearing water treatment. However, it is not possible to control temperature in cage rearing, resulting in a longer rearing period to market size, or the necessity to stock larger juveniles. On average, larger pre-fattened gilthead seabream (10 g) reach first commercial size (350-400 g) in about one year, while smaller juveniles (5 g) reach the same size in about 16 months.

Feeds are distributed by automatic feeders at 2-hour intervals from 08.00 to 20.00 for small fish (1-3 g), using increasing percentages of artificial feeds composed of 150-300 µm particles, or by hand for larger fish. Grading is necessary at least two or three times per cycle, in order to avoid growth differentiation. Fattening can occur in tanks or in cages system.

Before harvesting some days of starvation are needed. The length of this period varies, according to temperature and feeding rate (for example, at 25 °C, 24 hours may be enough). At lower temperatures, 48-72 hours are necessary. After correct starvation, the fish are ready to be harvested. Before starting this operation, the presence of dying or dead fish needs to be checked.

It is possible to harvest the fish in all weather conditions in land-based installations. In concrete tanks, workers push the animals toward the water inlet using a small trawl; then they remove them with dipnets or by pumping. Great attention is paid to cleaning the bottom of the tank before harvesting; this assures more hygienic fish and ensures that they have good organoleptic characteristics, as it avoids undesirable materials entering the gills and mouth.

Sea cages can be harvested when weather conditions are acceptable for the safety of the workers. Fish must be crowded into a relatively small area so that the animals can be harvested with dipnets or vacuum pumps.

In both sea cages and in land-based installations, gilthead seabream are normally killed by thermal shock; fish are put into tubs (plastic or stainless steel tubs) with iced water, which should be saturated with CO2, in order to reduce their suffering. Care is needed when taking the fish out of the water to pack them. This should be done rapidly to prevent scale loss and to preserve the appearance and brightness of the skin.

Production costs for 2 g juveniles in Italy vary from EUR 0.10-0.18/fish, depending on the fattening system. Costs for 5 g fry are about EUR 0.26-0.28/fish. The rearing costs to produce 350 g gilthead seabream range between EUR 3.0-4.0, depending on the rearing system.