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Species: O. mykiss Walbaum, 1792
Rainbow trout Oncorhynchus mykiss Walbaum (photo http://pond.dnr.cornell.edu)
The rainbow trout has an elongated body, the length of which is generally 5 times greater than the height. The head has a conical shape and the mouth is slightly oblique, with the maxillary bone extending to the posterior edge of the eye. The teeth of the trout are arranged in 1 or 2 series and are present only on the plow's stem. The lateral line is almost horizontal and on the back, in front of the caudal fin, there is an adipose fin.
The body color of the rainbow trout varies according to the habitat, the size of the specimens and the relative stage of sexual maturation. Generally the body of this salmonid has a blue-greenish hue in the dorsal portion, while the area of the hips shows a pigmentation that gradually tends to lighten and take on silvery reflections. On the sides there is a more or less high band of a purple pink color, which extends from the operculum to the caudal peduncle and takes on more intense shades tending to iridescent when the breeding season occurs.
The belly of the trout has a further lighter, almost whitish color.
Biology and habitat
Rainbow trout is a species native to North America, which over the years has been widely introduced in most of the planet's waterways.
In the area of origin there is a particular strain, called "Steelhead", which is characterized by anadromous behavior, similar to that of salmon. In fact, this species spends a few years of life in the sea and returns to the fresh water of origin when the reproductive phase begins. These animals are characterized by a very rapid growth and in just 3 years of life they are able to accumulate from 7 to 10 kg of weight, while normal rainbow trout, in the same period of time, increase by a maximum of 4.5 kg.
Rainbow trout was introduced to Italy about a century ago and as a consequence of its marked ability to adapt to multiple environmental conditions, the presence of this salmonid is nowadays found in most of the rivers and lakes present in the national territory. It is a species that is easily reproduced in conditions of captivity and is characterized by rather short growth times. The rainbow trout fry are larger than most fish species and this entails the possibility of being able to administer the artificial diet during the first breeding phases, thus shortening the time required for weaning.
This species adapts very well to various temperature conditions and can survive in environments characterized by a thermal range between 0 and 27 ° C. Trout growth occurs in waters with temperatures that can vary from 6 ° to 20 ° C with a thermal optimum between 14 ° and 18 ° C, while as regards the reproductive activity, the optimal thermal levels are between 10 ° and 12 ° C.
The maturation of the sexual organs generally takes place upon reaching the 3rd-4th year of age but by acting on the diet and on the thermal conditions of the environment, this can be anticipated. The trout eggs have a diameter ranging from 3 to 7 mm and the females can also produce up to 2000 for each kg of body weight. In breeding, by modifying some environmental parameters or through hormonal treatments on the reproducers, it is possible to carry out some manipulations in order to obtain only females or sterile individuals. The latter demonstrate a development similar to that of females, which is faster than that of males.
The laying of eggs in a natural environment occurs in the period from autumn to spring but in breeding this phase can be shifted over time, acting on the feeding, on the conditions of the photoperiod or through hormonal inductions.
In nature, rainbow trout feeds on insects, molluscs, fish eggs, small fish and crustaceans, including crayfish. The latter contains a carotenoid pigment responsible for the orange-pink coloring of the meat of those trout that are called "salmonate". This characteristic is very appreciated by the consumer and in breeding this coloring is obtained by adding two synthetic pigments to the fish diet, namely astaxanthin and canthaxanthin.
Trout breeding is generally practiced within intensive structures, where there must be the possibility of constantly being able to take advantage of high quality waters.
Trout is very sensitive to the presence of nitrogenous substances that derive from the catabolism of proteins such as ammonia, nitrates and nitrites. A parameter that must always be kept under control in trout breeding plants that use water pumped from underground wells, is that of the gas over-saturation of the water, which must be avoided as it causes the formation of gas bubbles in the blood of fish.
Trout are animals that are more tolerant of alkalinity and water hardness than acidity but in any case it is good practice to use neutral or alkaline water with a pH between 6.5 and 8.5.
The male and female breeding animals, after being selected, are housed in separate structures depending on the sex. The choice of their number depends on the size of the fry requirement and the ratio that is most frequently adopted between males and females is 1: 3.
The females are unable to lay naturally in a captive environment and when they are mature, human intervention is necessary to release the eggs. The trout are subjected to an anesthetic treatment and the eggs are then extracted, which is carried out manually by applying light pressure on the abdomen of the females. To reduce the stress of the animals, another extraction method has been devised, which uses the introduction of very low pressure air by inserting a needle inside the abdominal cavity, near the anal fin. The insufflated air facilitates the expulsion of the eggs and is subsequently removed from the female body by massaging the hips.
The sperm is collected inside containers by squeezing and is then added to the eggs, with an optimal ratio of 1-3 ml of seminal fluid for each liter of egg. Then the eggs are stirred and water is added to perform the "hardening" phase.
In this first stage, viable eggs (defined as green) are placed inside hatchers (hatchers), while non-viable eggs, which take on a white color, are removed. The time needed for hatching varies according to the water temperature. With a temperature of 3.9 ° C it takes 100 days, while with 14.4 ° C it takes 21 days, for a total of about 370 degrees / day.
The larvae manage to reabsorb the calf sack in a period ranging from 2 to 6 weeks, depending on the thermal conditions and are kept in special trays or troughs. When 2/3 of the calf sack has been absorbed, the feeding of artificial food begins with frequent meals and properly formulated diets.
When the fry reach a weight of 500 fish / kg, you can proceed with their transfer inside the pre-fattening tanks, where you generally operate with a density of 25 kg / m3.
The efficiency of the water exchange is very important during this phase, therefore if there is a situation of scarce water availability, it is advisable to reduce the load of the animals in the tank up to 10 kg / m3, in order to ensure at least 6 changes water per day. During this period and also in the following phases, the fish calibration operation is extremely important with the consequent formation of uniform groups by size, in order to facilitate management operations and reduce losses due to cannibalism.
The fry remain inside these structures until the ossification of the cartilage tissues is completed, which generally takes place around the 12th week of life.
Fattening techniques - Intensive breeding
The moment the fry reach a body length of 8-10 cm, they are transferred to the fattening structures, such as concrete raceways, artificial basins and cages. Raceways are the most used in traditional trout farming and are rectangular tanks that have a width of 2-3 m, a length between 12 and 30 m and a water body level of about 1 - 1.2 m. The amount of food that is administered, the amount of water exchange and the amount of oxygen to be integrated into the water body, must be commensurate with the biomass raised and the average size of the trout. The sustainable load inside the tanks must be established according to the environmental conditions and the water temperature.
Trout farming is carried out almost entirely with the use of artificial diets and the feed that is generally used is the extruded type.
The factor that has the greatest influence on trout growth is the water temperature, which inside the tanks should never deviate too much from the optimal thermal levels. In the presence of temperatures between 10 ° and 14 ° C, the trout can reach a weight of 80-100 g and a length of 15-20 cm in just 6-8 months from hatching and using high energy diets, these they reach the marketing size (250-300 g) in 10-12 months.
If you intend to produce "salmon trout", the animals, in the period prior to the sale, must receive specific feeding for a period of at least 45-60 days. If the trout are destined for processing, the fattening period requires time proportional to the desired size.
The breeding of rainbow trout in cages is carried out within floating structures, which use the water from existing water bodies and therefore allow a reduction in the initial investment capital compared to systems in ground tanks. These structures, however, are much less used than traditional ground systems.
Productions and market
Rainbow trout sold as a consumer product is marketed in many forms, that is, as a fresh product, as fillets, frozen, canned and smoked. Most of the production is destined for the fresh market and the marketing sizes vary according to the country in question. In the USA. trout are placed on the market when they reach the weight of 450-600 g and those with white meats are more appreciated. In Europe, the average consumer prefers large salmon trout, generally from 1 to 2 kg.
A considerable part of the production of rainbow trout in countries such as the U.S., Europe and Japan is also destined for the live market, both for the restocking of waterways and for sport fishing ponds.
- FAO. © 2005-2012. Cultured Aquatic Species Information Program. Oncorhynchus mykiss. Cultured Aquatic Species Information Program. Text by Cowx, THE. G. In: FAO Fisheries and Aquaculture Department [Online]. Rome. Updated 15 June 2005. [Cited 15 June 2012]. http://www.fao.org/fishery/culturedspecies/Oncorhynchus_mykiss/en
- CATAUDELLA S., BRONZI P. (2001). RESPONSIBLE AQUACULTURE Towards the aquatic productions of the third millennium. The bred species. Chap. 4.1, Pp. 499-505.
- MANZONI P., TEPEDINO V., copyright Eurofishmarket (2008). LARGE ILLUSTRATED FISH ENCYCLOPAEDIA. Guide to the recognition of over 600 species present in European waters or imported to European markets.
Card edited by Lapo Nannucci