Gilthead seabream belongs to the class of the Ray-finned fishes, to the order of the Spariformes and to the family of the Sparidae. Their origin lays in the Mediterranean and in the Eastern Atlantic from the British Isles to the Cape Verde and the Canary Islands to the coasts of Senegal. Apart from the sea, Gilthead seabram can be found also in the brackish water of estuaries.
Adult animals reach an overall length of 26 to 70 cm and a weight of 300 g to 4 kg. Body length and weight are hereditary. Gilthead seabream get 12 years old at the maximum; they reach sexual maturity in the first or second year of their life. The role of body’s colouring for mating is not clear; anyhow the intensity of the colours increases from July to October, then fading again in males while persisting in females until spawning.
The spawning season lasts from October to March with a peak in December and a tail towards the summer, depending on environmental conditions. Females spawn in large groups. Over a couple of days or even months they spawn ten thousands of eggs daily the weight of which corresponds to half or even twice their body weight. In captivity spawning can be triggered also in other seasons by raising water temperature.
Gilthead seabream are hermaphrodites that first evolve as males, later either morphing into females or living on as males. Sex conversion can also be induced by adding young animals to the group which causes older males to convert into females.
Gilthead seabream linger at the surface and in depths to 30, rarely 150 metres; by day they swim deeper than at night. They grow better in water temperatures around 25 °C, whereas temperatures below 11 °C or above 30 °C do not agree with them. For the summer season they migrate to bays and estuaries. When around winter the temperatures are decreasing they move back to the open sea where they were born. Aside from this seasonal alternation Gilthead seabream live relatively sedentary with a daily radius seeming to be less than 1 kilometre.
Gilthead seabream are rather diurnal and move around in schools, swimming less at night and eventually even burying themselves in the sand for rest. Big individuals however behave exactly contrariwise, being most active at night, presumably as they are solitary. Typically Gilthead seabream swim with a velocity of half a body length per second; on the run they can swim up to seven times as fast.
Gilthead seabream seem to like seaweed bottom best; yet they can also be found on sandy, gravely or rocky grounds or in estuaries with salt marshes or muddy bottom. They are mainly carnivorous, feeding predominantly on krill and worms and occasionally fry, less frequently on algae and other plants. In winter they feed in the dusk, in summer in the morning or towards evening. They fetch their prey by carving their head partly into the bottom.
Gilthead seabream are sensitive to light; manipulation of day/night-rhythm disagrees with them. They communicate through knocking and pulsed sounds produced by contracting sonic muscles that excite their swim bladder. They either live in small groups with linear hierarchy or in big anonymous schools; big individuals live also solitary. Dominant animals occupy the area of food and show more activity and aggressivity than the subordinates who either escape upwards or hardly move and stay away from food.
First extensive farming of Gilthead seabream existed in Italy probably already in the pre-Roman period. Evidence for such sites dates back about thousand years. In 1425 an official document of the Republic of Venice mentioned for the first time the still existing farming practice in so-called "valli". These valleys are pond-like partitions maintained for centuries in the lagoons alongside the coast, previously in the northern Adriatic, later in similar landscapes of France, Spain, Portugal or Egypt.
The extensive farming takes advantage of the seasonal migration of the species: In spring, the fishes swim into the sites, staying trapped there until slaughter.
Only in the 1980s Italian aquaculturists succeeded to artificially hatch juveniles on industrial scale. Thereby farming became independent of natural variability. As Gilthead seabream can be adapted to intensive farming in net cages along the shore seemingly without problems, the production of the favoured fish increased rapidly.
Today Gilthead seabream is one of the most frequently farmed fish species in the Mediterranean. Surprisingly because intensive farming of this species in question is hampered by at least three problems. Besides the viral and infectious diseases (bacterial, fungal, parasitic) as well as nutrient deficiencies that go beyond the scope of the FishEthoBase, we want to mention the “winter disease”.
Gilthead seabream are most sensitive to temperatures below 11 °C. In winter they withdraw to the open sea in order to escape falling inshore temperatures. Farmed fish however are compelled by ponds or net cages to stay in coastal waters of decreasing temperature. Cool temperatures are one of the reasons for the "winter disease" affecting Gilthead seabream mainly in the northern Mediterranean, in fact up to 60 per cent of the animals in their first year of life. Sick animals get lethargic, swim abnormally, starve, have their immune system compromised; finally they die.
A species-appropriate measurement would consist of preventing lower temperatures and bacteria Pseudomonas anguilispetica by adequate site selection. Closed containment recirculation systems (RAS) could avoid both disease factors (not yet tested to our knowledge).
The second problem we want to mention already starts within the first days of artificially hatched Gilthead seabream. Of the larvae, 8 to 100% develop skeletal deformities or morphological malformations. Because these larvae do not gain the same weight as normal ones, they represent a „commercial loss“ and are eliminated as soon as they are detected. This means that intensive farming of Gilthead seabream is based on a hatchery industry that calculates with a high waste of animals which is ethically questionable.
Note of dissent:
Reviewer Ana Roque does not regard this practice as ethically questionable but as an issue of the business plan. She holds the view that probably similar proportions of deformities exist in wild stocks and as long as there are no validated alternative protocols she would not blame the hatcheries.
The author as well as reviewer Jenny Volstorf hold on to the statement reminding that aquaculturists usually like to point out that they manage to grow the overwhelming majority of eggs to fish, in stark contrast to wildlife’s inefficiency.
To date, there is no alternative in sight. Stocking with wild juveniles, as done in small-scale extensive farms in the past, is no longer an option since farming has reached industrial scale. Meeting steadily increasing demands for juveniles with wild stocks would jeopardize the survival of wild populations.
A similar reflection emerges related to the feeding of Gilthead seabream who by their nature feed on krill mainly. Farmers will have to decide whether or not to rear a carnivorous species that is fed wild fish. This does not affect the target species alone but impairs the marine food chain and consequently the living of marine animals, thus animal welfare. Not to mention the welfare of about 450-1000 milliard* fishes caught for feed annually. As more and more consumers become aware of these matters, aquaculture of omnivorous or herbivorous species could prove to be the better choice in the very near future.
* Milliard in FishEthoBase = a thousand millions
Note of dissent:
Reviewer Ana Roque disagrees with this statement which to her opinion has to do with overfishing and ethics but not with animal welfare.
The author as well as reviewer Jenny Volstorf adhere to the statement because overfishing impairs the marine food chain and consequently the living of marine animals, thus animal welfare.
Based on research so far, further questions remain unanswered. Juveniles in stocking densities of 22 kg per cubic metre or higher displayed more stress than individuals in control densities of 10 kg per cubic metre or lower. The stress-commencing threshold, thus, could be well below 22 kg per cubic metre. Furthermore it is clear that Gilthead seabream should not be kept in groups of less than 75 animals; but there are still no satisfying scientific findings on how to design a species-appropriate artificial habitat for them.
Labels are not helpful for clients who care for the welfare of farmed fish. The guidelines of organic labels are the ones most inclined to grant animal welfare, yet they define no tangible instruction. All other labels address animal health at best, but do not acknowledge all-encompassing aspects of animal welfare. That is to say that even fishes farmed under labels like organic, ASC, or Friend of the Sea, often live under the conditions of intensive animal husbandry.
If we want to change the disregard for animal welfare, we need more of two things: ethological research and pressure from concerned consumers who want to eat respectfully-farmed fish.
Billo Heinzpeter Studer, 05.06.2015 © fair-fish international
Sources: FishEthoBase, Findings and Recommendations, and FishBase.org
Reviewed by Ana Roque and Jenny Volstorf
Cite as: "Studer, Billo Heinzpeter, 2015. Gilthead seabream (Summary). In: FishEthoBase Research Group (editor), FishEthoBase. World Wide Web electronic publication. www.fishethobase.fair-fish.ch"