Species group: Salmon and Trout
Other common names:
Scientific name: Oncorhynchus mykiss
Originally native to the cold water rivers of the Pacific coasts of Asia and North America, the rainbow trout has been introduced to rivers in Europe, South America, Central Africa and Australia as a sport fish. It is a good river fish as it can tolerate a high range of temperatures and water conditions and it grows rapidly. They need clear (non-turbid) water to hunt and will eat a range of food from insects and insect larvae to crustaceans and water snails. The adults are far more tolerant to varying water quality, pH and oxygen levels than the eggs, fry and immature fish and this is why European techniques invented in the 1830s to keep brown trout on a small scale to supplement farm incomes were extended to the large-scale production of rainbow trout fry (the first hatchery was established in San Francisco Bay in 1870). Originally the fry were used to stock fishing rivers, as rainbow trout grew to valuable size and weight much quicker than native European brown trout. Though rainbow trout are still reared as stockfish for game fishing, today, they are also one of the most common aquaculture fish for sale as food. There are two types of rainbow trout, those that remain in freshwater all the time and those that are anadromous (they migrate to the sea and return to freshwater to spawn). These saltwater forms of rainbow trout are known as 'steelhead' in North America and Ocean Trout in Australia. Rainbow trout lend themselves both to large-scale and small scale aquaculture and an industry that has been established in America and Britain for over 50 years is now rapidly establishing itself across Eastern Europe. Today over 800 000 tonnes of rainbow trout are farmed worldwide. The largest producers are Chile and Norway, though they tend to farm the ocean-going version of the Rainbow Trout, the Steelhead, in ocean cages. The largest producers of freshwater rainbow trout are Italy, France, Germany, Denmark and Spain. In the growing tanks the fish are typically quite calm and when someone looks in, they appear almost still. This is because water is constantly flowing over them, quite fast, and the fish are facing upstream and swimming against the current. When moved to their storage tanks the fish are more active and can be seen swimming and jumping. Kids love it when they are fed as the surface of the water turns into a mass of writhing fish. Freshwater rainbow trout spawn only in running water which must have a pH over 6.5. For farming, a process known as artificial spawning is used. Mature fish (typically three to four years old) are caught from brood pools, the females are milked under anaesthezia by squeezing by applying pressure from the pelvic fins to the vent area to eject their eggs to a fertilizing dish. The males are then milked to release their sperm on top of the eggs (typically one male to three females is the ratio used). The fertilized eggs are moved to hatchery tanks that are typically shallow troughs with just a trickle of water to aerate. Dead eggs are removed and the eggs are allowed to develop, undisturbed, until clear eyes can be seen in the developing embryos. Dead eggs are removed and the fry are allowed to hath. Hatched fry are moved to rearing tanks (typically like animal feed troughs that have a water inlet for aeration at one end and a water outlet at the other). They are fed fish food and allowed to grow until they are of fingerling size. Then they are moved to rearing tanks. These are large tanks, typically set on a gradient so that you can have several tanks fed by a single water supply with the outlet from one tank being the inlet to another.
The shape of the fish resembles that of a salmon overall. The ground colour of the scales are green and they have a distinct pink lateral line. The fish has a broad reddish band, often associated with other colours on the sides and gill covers (this is why they are called rainbow trout). They also have distinctive dark spots on the caudal and dorsal fins and the sales are typically mottled with dark spots. When ready to mate the males become redder in colour.
15cm long, 900g in weight at time of sale (they can grow up to 120cm long, 24kg)
Rainbow trout need fast flowing water. The rate of flow typically should not exceed 3/4 the length of the fish. So a 5cm long fish should have water that flows no faster than 4cm/sec. As each growth stage of the fish requires different water temperatures and flow rates, the fish are moved to larger and larger tanks as they grow and the water in those tanks flows faster. When they reach marketable weight (anything from 500g upwards) they are typically moved outside to storage tanks ready for sale. This would be a the stake that the fish would be released if being raised for game fishing purposes. Historically, rainbow trout were fed minced abattoir offal. Today, they are typically fed commercial, pelleted, trout feed. With the type of feed gauged to the size of the fish. Fingerlings are fed the smallest grade of food some 6 to 8 times per day, growing fish are fed medium-grade food some 4 to 6 times per day. Once the fish reach size they are often moved to an outside pool and are fed the largest grade of food some three to four times per day. Rainbow trout are typically healthy and their preference for fast moving water tends to minimize their chance of developing parasitic diseases. Each time a tank is emptied it should be scrupulously cleaned and sterilized before new fish are introduced. Bacterial and fungal infections of the gills rare the most common diseases and these can be minimized by good sanitation and arranging tanks so that outflow and inflow are not connected. Some diseases can be treated using antibiotics, and there are vaccinations for others. Copper sulphate is traditionally used for parasites. There are viral diseases as well and for these there are no treatments except to eliminate infected stock.