by Rebecca Sherratt, Production editor, International Aquafeed

Rainbow trout (Onchorhynchus mykiss) are freshwater fish, that can be identified by their broad, purple stripes that run along their flanks. They are native only to North American rivers and lakes, but their popularity has led to expansion in every other continent, with the exception of Antarctica. They are silvery-brown fish, that can be recognised by their dark spots. This fish can grow between 50-70cm long and weigh up to 14kg. Their average lifespan is four-to-six years. Their growth rate differs, based upon the water temperatures they reside in.

Rainbow trout comes in a variety of consumable products, ranging from being served fresh, smoked, whole, filleted or canned. It can also be eaten boiled, broiled, fried and baked. Rainbow trout farming has been taking place for several hundred years, and only continues to expand. The systems currently being used are well-established, but development is continually being done to enhance the production efficiency and sales of fish. Genetically-modified hormones have proved effective in enhancing the production rates of rainbow trout, but many issues prove to arise, when on the topic of genetic modification in fish farming.

There are migratory rainbow trout, that leave the rivers and enter the ocean. These are anadromous, or nicknamed "spearheads", due to the silvery markings they obtain. Rainbow trout eat eggs, insects, crustaceans and very small fish. Their natural enemies are larger fish, racoons, eagles and herons. They are not considered endangered in any way, and over 15 subspecies of rainbow trout. are native, just in North America alone. Various subspeciesinclude eagle lake trout, golden trout, beardslee trout and the kamchatkan rainbow trout, to name a few.

Female rainbow trout form nests for their eggs in gravel, and lay egg clusters of between 200-8,000 eggs at a time. Fertilised eggs are then left on their own, for four-to-seven weeks, to incubate. Newly-hatched rainbow trout feed on the remnants of their yolk, during the first two weeks they are alive. Following this period, they devour zooplankton.

Juvenile rainbow trout are known as "parr", or fingerlings, and develop dark, vertical stripes along their bodies. At three or four years old, they reach sexual maturity, and spawn in the very streams they were born in.

A nutritious meal

Humans have come to consume a vast quantity of rainbow trout, due to their delicious meat becoming popular as a rich source of omega-3 fatty acids and vitamin B. Rainbow trout is especially popular in Western cuisine, with its nutty flavour.Since the 1950s, rainbow trout production has increased drastically, especially in Europe and Chile. Primary producing countries include Italy, France, Denmark, Spain and Germany.

Trout eggs are artificially spawned into aquaculture systems, in a variety of production numbers, depending on the needs of the fish farm and its schedule. The most commonly used sex-ratio is one male for every three females for broodstock, and the genders are usually kept separate. Over population of broodstock can be an issue if they fish numbers are not correctly controlled, which proves quite costly.

The most common method of reproducing rainbow trout is through the dry method, which involves eggs being removed from the females (who are subject to anaesthetic), by applying pressure to the pelvic fins and the vent area. Up to 2,000 eggs per kilogram of body weight are collected in a dry pan, and they are kept dry to enhance the fertilisation.

Males are then stripped in a similar way- their milt is collected in a bowl, to avoid water and urine contamination. This is then mixed with the eggs, along with water. This activates the sperm, causing the eggs to increase by 20 percent in size- a process known as water-hardening. After 20 minutes, fertilised eggs can be transported.

Monosex culture has been growing in popularity in recent years. This is where females, or triploids, (fish with three sets of chromosomes, rather than the usual two), are housed separately, with no males, to improve production output.

Vertical-flow hatching troughs are used to incubate the eggs, once fertilised. These are usually between 40-50cm wide, 20cm deep and up to four metres in length. Eggs remain in this stage, until they reach the eyed stage. As the eggs begin to hatch, at between four-to-14 weeks, the fry drop through the mesh of the hatching trough, into a bottom trough. The hatching time for rainbow trout fry varies depending on water temperature, taking roughly 100 days at 3.9°, and roughly 21 days at 14.4°C.

Dead eggs are removed regularly, to minimise the risk of fungal infection. Infections can also be controlled through the use of formalin, (a 37% formaldehyde solution). By adding this solution to the inflow water at 1:600 dilution for 15 minutes each day, risk of infection decreases significantly.

Fibreglass or concrete tanks are the storage methods of choice, when fry are reared. These are preferably circular in shape, in order to maintain a regular current of water and uniform distribution of fry.

Trout farms

A Danish entrepreneur first introduced trout farming to the UK in the 1950s.Rainbow trout farms are the most the most popular farms in the UK, as their climate proves most complementary for the farming of trout. 16,000 tonnes of rainbow trout are produced in Britain every year, most of which are farmed in freshwater tanks, ponds, netting cages and raceways. Some of these will be farmed in sea cages.

The pH levels of the water rainbow trout are comfortable to reside in varies, depending on their stage in their growth cycle. For developing embryos and fry, a pH level of 6.5-8 is desirable whilst their tolerance of a more varied increase in pH level increases as the trout grow older.

The same is also true for the optimal water temperature which rainbow trout flourish within. For trout embryos and fry, 10°C is ideal, whereas for an adult rainbow trout, any temperature ranging between 7-18°C is also ideal. The temperature of the water also directly correlates with the appetite of rainbow trout, so if your trout seem to not be eating enough, then unsatisfactory water temperatures may be the cause.

Planning the size of your rearing devices and production unit for rainbow trout is often done in a reverse method. Rainbow trout grow rapidly, and as a fairly large fish, it is wise to first plan and estimate the final size of your fish, to then calculate the size required for your production unit. The density of your fish must also be taken into consideration.

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