Rainbow trout Oncorhynchus mykiss; Walbaum, 1792. Family: Salmonidae
Rainbow trout (Oncorhynchus mykiss) is a Pacific salmonid species that was first brought to Britain in 1884. Fast-growing and tolerant of crowding in captivity, they are now widely used around the world for fish farming and restocking of angling fisheries. Rainbow trout have a prominently spotted tail and often a wide band of red, pink or mauve along the flanks.
The original source populations in North America were a mixture of stream-living, lake-dwelling and sea-going forms (steelhead) and most spawned in late winter. Freshwater forms feed on invertebrates and fish.
Rainbow trout in common use today come from domestic strains developed to suit intensive fish farming. When used for angling, most are stocked at a takeable size for rapid removal (put-and-take). Stocking densities and average weights are dictated by the economics of angling demand and can be much greater than the biological carrying capacity of the waters concerned. Relatively few fish are capable of surviving for longer than 12 months at liberty. Due to the now widespread use of all-female or sterile triploid strains, spawning is uncommon. There are no known self-maintaining populations of rainbow trout in Scotland, and very few in England. More than 300 Scottish stillwaters are stocked with Rainbow trout. Many of these fisheries are ponds of only a few hectares in area. Rainbow trout that have escaped from fish farms or migrated from inadequately screened fisheries are often found in rivers and some have been known to visit the sea.