Order: Salmoniformes Family: Salmonidae Subfamily: Salmoninae
Foto: Lauri Urho
Description: The name rainbow trout refers to the broad reddish band with a wash of other colours on the sides and gill covers of the species. In appearance, the rainbow trout is not unlike a rather sturdy brown trout. It can be distinguished from all other salmonids by the pink lateral band, and by the dark spots on its dorsal and caudal fins. It is still sometimes referred to by the Latin name Parasalmo mykiss.
Origin and distribution: The rainbow trout is native to rivers and streams draining into the Pacific Ocean in Asia and North America, south of 60 degrees latitude. In Asia, it occurs in the Kamchatka Peninsula and locally within the Sea of Okhotsk area, and in North America as far south as the Sierra Nevada. Throughout its range, three ecological forms are known: the anadromous rainbow trout, or steelhead; the lake form, which thrives in cold lakes; and the river form, which spends its whole life cycle in rivers. The lake form is used for aquaculture and has been distributed all over the world. The first rainbow trout eggs were imported into Finland from Germany in 1894, and the first experimental stockings into wild waters took place in the early 20th century. Only since the 1960s have stockings been carried out on a more regular basis, resulting in sizeable catches. Rainbow trout have been released into wild waters throughout almost the whole country. Fish that have escaped from rearing cages are found in the sea, notably in the Archipelago Sea and Åland Sea.
Reproduction: Rainbow trout reproduce in running water, though permanent self-sustaining stocks do not commonly occur in Finland. To meet the reproduction requirements of the species, the pH value of the water must be over 6.5. Rainbow trout larvae born in the wild have been found in southern Finland, but there is no evidence of reproduction of successive generations in general.
Food, growth and migrations: Rainbow trout larvae feed first on zooplankton and thereafter on water insects and their larvae and also on insects landing on the water surface. Later they turn to a diet of fish. When released into wild waters as juveniles, they grow somewhat better than brown trout (Salmo trutta) but more slowly than Atlantic salmon (Salmo salar). When released at catch size (0.5 – 2.0 kg), as is currently common practice, they do not usually grow much any more. Rainbow trout released in flowing water tend to be caught near the release site. When released in lakes or the sea, they make lengthy migrations. For example, catch-size rainbow trout released in the Gulf of Finland in 1994 were eventually caught in the southern Baltic Sea.
Fishing and catches: Rainbow trout are released at catch size to increase the probability of catches for rod anglers. Such fish are usually caught rather soon. The catch by recreational fishermen was about 0.6 million kg in 2006. The rainbow trout is the most important aqauculture product for human consumption with about 15 million kg per year. Early production takes place mostly in fresh waters and the main food production in coastal net cages.
Vulnerability, threats and management: It has been claimed that rainbow trout compete with native fish species, primarily brown trout. There may indeed be competition for food resources, but nowhere in Finland have rainbow trout been shown to displace a native species. Nevertheless, the potential risk exists.