Rainbow trout, whose name refers to the many rainbow-coloured spots on its skin, is the leading freshwater cultured species in Europe. Native to the Pacific coastal area in the United States, it was introduced into Europe at the end of the 19th century. Its hardiness and fast growth quickly proved particularly well adapted to aquaculture. Rainbow trout are farmed today in nearly all European countries, especially in coastal countries with a temperate climate.
Latin name – Oncorhynchus mykissProduction (EU-27) – 204 745 t (2007), 31 % of global production.Value (EU-27) – EUR 539 million (2007).Main EU producer countries – Italy, France, Denmark, Spain, Germany.Main producer countries worldwide – Chile, Norway, Turkey, Iran, USA.Fact sheet
Rainbow trout have difficulty spawning naturally in Europe because the female cannot lay eggs in autumn. Fish farms therefore keep a wide range of broodstock on hand to avoid a shortage of eggs. In the past, such shortages have obliged European farms to import eggs.
The females are rarely used for reproduction before the age of three or four years. Dry fertilisation is the method used most frequently. The eggs are removed manually by applying pressure. The milt of several males is mixed with the eggs. Eggs fertilised in this way can be transported after 20 minutes and up to 48 hours after fertilisation.
The trout larvae are reared in fibreglass or concrete tanks that are circular in shape to maintain a regular current and uniform distribution of the larvae.
The larvae hatch with a yolk sac that contains the food they need for their initial development. Once the sac has been absorbed, the fry swim up to the surface to look for food and the air they need to fill their swim-bladder for the first time. They are fed a diet of small flakes containing protein, vitamins and oils. Hand feeding is preferred in the first stages of rearing to avoid over-feeding. The fry are then fed small pellets until they reach a weight of 50 g and are 8 to 10 cm long.
The young fish are then transported to grow-out units, either floating cages in lakes or, most often, tanks located beside a river. These tanks, generally rectangular in shape and made of concrete, operate on two techniques: flow-through, an open system where river water flows through the units via a race; and recirculation, a closed system that consists of circulating water in the tanks and recycling it via pumping and processing units. The advantage of recirculation is that the water temperature can be controlled, which means winter production can be maintained in central and eastern European countries.
Trout are carnivorous and need a high-protein diet. In the right environment, a trout can grow to 350 g in 10 to 12 months and 3 kg in two years. Fattening farms using floating cages are also used at sea, in the low-saline waters of the Baltic and in the protected waters of the Scandinavian fjords. Sea trout are fed a diet similar to that of salmon, which accounts for their salmon-coloured meat. During fattening, good stock management involves separating fast-growing trout from the others: the stock is usually sorted four times during the production cycle. When the fish have reached commercial weight, the trout are collected with a net in the tanks or pumped live if they have been reared in cages.
Rainbow trout are available in Europe year-round.
For weights of up to 400 g, they are available white or salmon-coloured, whole or filleted, fresh or smoked. If they are reared a bit longer and reach a weight of 1.5 kg, rainbow trout are sold, like salmon, fresh (filleted or as fish steaks) or smoked (sliced). Salted trout eggs are also eaten, especially in northern Europe.
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