Flatfish, especially the larger species, can attract a high price, and their firm, meaty texture make them a favourite on restaurant menus. They are also a great fish to eat at home, with smaller fish cooked whole, and larger ones prepared as fillets. Either way, they are particularly tasty!
There are more than 700 species of flatfish, and the ASC Flatfish Standard covers some of the most important commercially produced ones, which are Halibut in the genus Hippoglossus, Flounder in the genus Paralichthys, and Turbot in the genus Scopthalmus.
The term flatfish refers to fish that have two eyes on one side of their face. Interestingly, they start life as tiny round fish, but as they grow, their bodies start to flatten out, one eye migrates across the head, and the mouth twists to the side. Flatfish species are either right-eyed or left-eyed, depending on which way the eye moves.
Turbot are benthic (bottom dwelling) flatfish with small eyes and a large mouth, which live on sandy and muddy seabeds in shallow waters, down to a depth of around 100m. These fish are native to the Northeast Atlantic, Baltic Sea and the Mediterranean Sea.
Adult turbot feed mainly on other demersal fish, crustaceans and bivalves. Turbot are long-lived, surviving to around 25 years old, and can grow up to 25kg. Unusually for a fish, they don’t have scales.
Halibut are the largest species of flatfish and live on very deep seabeds, although they swim upwards to feed. Halibut are dark brown on the top side with an off-white underbelly, and are covered in very small scales. Juvenile halibut feed on small crustaceans and other benthic species such as octopus, crab, cod and flounder.
Halibut are distributed throughout the north Atlantic off the Norway coast, Faroes, Iceland and southern Greenland, although they can be encountered as far south as Maine in North America, and the Bay of Biscay, in cold waters between 5-8°C.
Flounder grow to between 22 – 60 cm and can live up to 15 years in the wild. They spend most of their lives in estuaries, but also swim out to sea to depths of 50 metres.
Adult flounder camouflage themselves on the seabed in soft, muddy areas to protect themselves from predators. Their diet mainly consists of fish eggs, small fish and crustaceans. Flounder became a candidate for aquaculture due to their reducing populations, caused by overfishing.
Flatfish are well suited to land-based farming and can be grown in a variety of tank types. Feed companies have developed special dry diets for flatfish, which the fish are weaned onto at an early stage.
Turbot aquaculture research began in the 1970s in Scotland and France, but the industry only became commercially viable in the 1990s, when advances were made in juvenile breeding techniques. Further development of turbot farming has been helped by investment in improved facilities, the production of dry feeds, and the introduction of vaccines for diseases that commonly affect turbot.
China is the largest producer of turbot, (50,400 tonnes in 2018), while in Europe, Spain was the largest producer with 7,995 tonnes, followed by France with 300 tonnes, and the Netherlands with 100 tonnes.
Halibut farming began in the 1980s in land-based systems and in marine net pens. However, the production of halibut has remained fairly low due to technical difficulties. In 2018, 1,918 tonnes of Atlantic halibut came from aquaculture, 96% of which was produced in Norway and the remainder in Scotland.
From egg to juvenile halibut takes around four months of careful nurturing. A further 3 to 4 years is needed before the fish reach market size of 3 – 5kg or 5 -7kg. Halibut need to be kept at a low temperature, fed on a specific diet, and protected from sunburn if they are grown in sea pens.
Olive flounder (Paralichthys olivaceus) is highly popular in Korea, China and Japan, which are the most significant land-based producers of this fish. Native to the north-western Pacific Ocean, Olive Flounder is a left-eyed flatfish.
This fish, which is more widely known as the Japanese flounder or hirame, is also considered to be a good future candidate species for the US aquaculture industry, thanks to its relatively short growout time compared with other flatfish species, efficient food conversion ratio, and well-established market presence.
Impacts of farming flatfish
The farming of flatfish including flounder, turbot, and halibut has been associated with a number of environmental impacts, including the use of marine ingredients in feed, negative impacts on biodiversity, use of antibiotics, disease and parasite transfer, and impact of pollution on water quality and the seabed.
ASC certification demands that farms adhere to strict guidelines to ensure they produce a healthy product, while minimising any negative environmental and social impacts associated with their operations.
ASC certified flatfish farmers work hard to improve standards in their industry, liaise with their local communities, and respect their workers and the environment.
ASC certified flatfish farms must minimise any impacts on their local ecosystems. They do this in a number of ways, such as the development and implementation of an impact assessment to protect wildlife and sensitive habitats, protection of the ecological quality of the seabed, and ensuring farms are not sited in High Conservation Value Areas (HCVA). Certified farms must greatly minimise any fish escapes so as not to affect local wild populations.
Use of wild fish as an ingredient in feed must be strictly limited and fully traceable back to a responsibly managed, and preferably certified source. Non marine ingredients such as soy and palm oil must also come from responsibly managed sources.
ASC certified flatfish farms are required to measure water parameters including phosphorous and oxygen levels at regular intervals, and they must remain within set limits. Potential copper release into the water must also be minimised and monitored.
Farms must also adhere to rigorous requirements to minimise any likelihood of disease and/or strictly manage outbreaks. Management plans must be put in place to deal with parasites, with the aim of reducing numbers, and plans to deal with mortalities are also required. Preventative use of medicines is prohibited.
All ASC certified farms are safe and equitable working environments where employees earn a decent wage and have regulated working hours. Any form of forced or child labour is strictly prohibited under ASC certification guidelines, based on the core principles of the International Labour Organisation (ILO). Certified farms must engage with local communities and have a system in place to inform them about potential health and safety risks.
Cooking with flatfish
The best and simplest way to cook all three species of flatfish is to pan fry them in butter, with a squeeze of lemon juice. This works well for smaller fish, which can be cooked whole, on the bone, to retain moisture and add to the flavour, as well as larger fillets.