Rainbow Trout, Oncorhynchus mykiss

Rainbow trout are classified as Oncorhynchus mykiss, and as such belong to the same genus as Pacific salmon, and to the family Salmonidae, which includes Atlantic salmon (Salmo salar), various trout (Salvelinus sp.), Arctic char (Salvelinus alpinus), Arctic grayling (Thymallus arcticus) and whitefish (Coregonus sp.). Rainbow trout are native to areas around the North Pacific Ocean, from southern California through Alaska, the Aleutians and the western Pacific areas of the Kamchatka Peninsula and Okhotska Sea drainages. Rainbow trout primarily inhabit fresh water, but in the eastern and western North Pacific anadromous stocks are found. These stocks follow a life cycle similar to that of Pacific salmon, in that they spend a part of their life in the ocean, but return to lakes and rivers for spawning and the fry and juvenile stages of their life history. Most strains of rainbow trout can adapt to life in sea water, once they reach the post-juvenile stage (c. 75–100 g), through a gradual increase in salinity of their rearing water. This is one of the qualities of rainbow trout that has led to their prominence as a farmed species of fish. Rainbow trout have been cultured for hundreds of years, and are the most widely farmed trout in the world. Rainbow trout can tolerate a wide range of water temperatures and other environmental variables, such as water quality, but they require highly oxygenated water and thrive in water temperatures of 13–18°C. They are a highly valued foodfish, and can be grown to have pigmented (red) or non-pigmented (white) flesh, depending upon their diet. In North America, Britain, Denmark, France and Italy, most trout farming occurs in fresh water, using flow-through water-supply systems. In Chile and Scandinavian countries, rainbow trout are grown in marine cages after an initial phase in fresh water. Diet formulations for rainbow trout grown in marine cages differ somewhat from those for trout grown in fresh water (Table 14.1). Global production of rainbow trout was 358,456 t in 1995, second only to Atlantic salmon among salmonids (Tacon, 1998). France, Chile, Denmark and Italy