Fish and sustainability is something I always question and consider to be undesriabe especially because in Spain they have the finest culture for Turbot: https://mesubim.com/2019/11/18/elkano-ultima-video/

The Stolt Sea Farm is a world leader in the production of turbot and sole and they produce 7500 tonnes of Turbot. The species is known as Psetta Maxima and is one of the most desirable of farmed fish due to its consistency. But consider one factor, and that is the fish are grown in cement tanks and fed various materials, so at the end it is rather disgusting.

Turbot aquaculture commenced in the 1970s in Scotland (UK) and it was subsequently introduced to France and to Spain. At first, the number of installations in Spain was rather limited due to the scarcity of juveniles. The technological development of juvenile production changes this. At the beginning of the 1990s, there were already 16 producers in Spain. A significant crisis in turbot culture occurred in 1992; there was an increase of 52 percent in production but the industry lacked a consolidated commercial marketing network.

Another factor that contributed to this crisis was that the farms were small and had very high production costs. This crisis caused the closure of some farms. From that moment onwards a reorganization of the sector began, which gave rise to a growth both in production and in the number of countries where turbot is farmed. Spain, with its highly suitable oceanographic conditions, is now the major producer worldwide but turbot is also currently farmed in Denmark, Germany, Iceland, Ireland, Italy, Norway, Wales (UK), and Portugal, and was previously reared in the Netherlands. The natural distribution of the turbot includes coastal waters of all these countries. Turbot has also been introduced to other regions (notably Chile in the late 1980s) and, more recently, China.

Besides commercial investment in improved facilities or the construction of new farms, other decisive factors have assisted in the consolidation and development of the sector. These have included the production of dry feeds and the development of vaccines for the most important diseases affecting turbot.

Psetta maxima is a benthic marine species, living on sandy and muddy bottoms, from shallow waters to 100 m. Younger individuals tend to live in shallower areas. Cryptic, imitating the colour of the substrate. Carnivorous, juveniles feeding on molluscs and crustaceans, and adults mainly on fish and cephalopods. Spawning (sequenced, every 2-4 days) usually takes place between February and April inclusive in the Mediterranean, and between May and July inclusive in the Atlantic. Eggs have a single fat drop. Larvae are initially symmetric, but by the end of metamorphosis (day 40-50, 25 mm) the right eye has moved to the left, giving rise to asymmetry. Formerly known as Scophthalmus maximus.

Psetta maxima is a gonochoric species with separate sexes. Broodstock are maintained in square concrete or cement tanks, with volumes ranging from 20-40 m³ at densities of 3-6 kg/m³ and fed on moist pellets. The spawn are obtained by stripping. Females undergo ovulatory cycles with an approximate period of 70-90 hours. The eggs are pelagic and spherical in shape. Egg diameter varies between 0.9 mm and 1.2 mm. Embryonic development takes 60-70 days. After hatching, turbot larvae are 2.7-3.1 mm in length.

Larval culture may be semi-intensive or intensive. In semi-intensive systems, larvae are cultured at low density (2-5 larvae/litre) in a large volume (50 m³), while in intensive culture larval density is higher (15-20/litre) and tank volume is 20-30 m³. In both systems the rearing temperature is 18-20 ºC. The newly hatched larvae feed from their vitelline reserves; mouth opening occurs on day 3. Feeding is based on rotifers and Artemia. Phytoplankton is added to the culture medium. Weaning is in round-cornered square tanks with open-circuit pumped seawater. Various commercial feeds are used at the weaning stage.

Turbot are nursed in square or circular tanks (10-30 m³) with open-circuit pumped seawater. Aeration systems are usually used to maintain the water at oxygen saturation. Juveniles are fed with dry pelleted feed, introduced manually or automatically. The weight range varies between 5-10 g and 80-100 g during the pre-fattening period (duration 4-6 months).

Turbot are either reared in on-shore tanks (the most common technique for this species) or flat-bottomed cages. Square or circular cement tanks (25-100 m³) are used, with open-circuit pumped seawater. Aeration or oxygenation systems are normally used to maintain the water at oxygen saturation. Feeding consists of extruded pellets, introduced manually or automatically. The elements that determine productivity are temperature and fry quality. The optimum temperatures for feeding range from 14-18ºC, while the extreme range for the culture of turbot is 11-23ºC. The limiting factors are pathology, culture technology and the market.

Cages submerged at various levels, or floating cages, in both cases flat-bottomed, are used. The frames are metal, with a metal or netting bottom. Extruded pelleted feeds are manually fed. The elements that determine productivity are suitable location and water temperature, and fry quality.

Commercial turbot feeds are available, with a current (2003) cost of EUR 900/tonnes. Typical FCR is 1.1-1.2:1. Fish are harvested manually and killed by placing them into containers filled with ice and seawater and transported to processing units.

Harvested fish are packed in polystyrene boxes, covered with a layer of ice and plastic film. In Spain, turbot are generally marketed whole and fresh, while in the rest of Europe they are generally gutted before sale. Spain has begun to produce filleted turbot to satisfy other European market demand. Size demand has changed. Formerly it ranged 1.5-2.0 kg but now smaller sizes are acceptable; currently sales range between 0.7 kg and 2.0 kg.

The ongrowing production cost is about EUR 5-6/kg in tanks and EUR 5/kg in cages. Despite the higher costs of on-shore tank culture, this remains the norm because cage culture of this species is still experimental stage and there are few locations that meet the optimum conditions for on-growing. And the application of slurry ice, a binary mixture of small spherical ice crystals surrounded by seawater at subzero temperature, is a potentially new preservation method for farmed turbot but nevertheless it is grotesque. (source internet)

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