RAINBOW TROUT [Onchorynchus mykiss.]
Although native to lakes and streams of North-West America, Rainbow Trout has been introduced to regions throughout the world including Australasia, South America and South Africa. It is now the main breed of trout to be farmed commercially. Rainbow Trout can be distinguished by the broad purple or violet band along its flanks and the black spots on the tail fin. There is a migratory breed of Rainbow Trout known as the Steelhead. Steelheads are anadromous fish which means they only visit freshwater from the sea to spawn. Rainbow Trout are also highly prized as game fish because they fight hard, leaping high out of the water.
The small black spots which are liberally sprinkled over the tail are a simple way of recognising the Rainbow Trout. These are not found on any other trout in Europe. The body is generally silvery with a pinkish zone along either side of the lateral line and both body and fins are sprinkled with the same small black spots.
The Rainbow Trout is the most adaptable of trout, both in its native California and in Europe. It lives in lakes, streams and rivers, and many populations feed in the sea, returning to the rivers to spawn.
Rainbow Trout can thrive in water that is too warm (up to 22°C) or too low in oxygen content for Brown Trout. Because of this hardiness and their high growth rate, Rainbows are the most popular fish for raising in fisheries.
BREEDING & GROWTH
Rainbow Trout, including Steelhead forms, generally spawn in early to late spring (January to June in the Northern Hemisphere and September to November in the Southern Hemisphere) when water temperatures reach at least (6 to 7 °C). The maximum recorded lifespan for a Rainbow Trout is 11 years.
Freshwater resident Rainbows usually inhabit and spawn in small to moderately large, well oxygenated, shallow rivers with gravel bottoms. They are native to the alluvial or freestone streams that are typical tributaries of the Pacific basin, but introduced Rainbow Trout have established wild, self-sustaining populations in other river types such as bedrock and spring creeks. Lake resident Rainbows are usually found in moderately deep, cool lakes with adequate shallows and vegetation to support production of sufficient food sources. Lake populations generally require access to gravelly bottomed streams to be self-sustaining.
Rainbow Trout spawn in redds cut in gravel by the female, in a manner generally similar to the salmon. Growth can be very fast, and under semi-natural conditions fish have reached over 3.6 kg in four years.
Rainbow Trout feed mainly on aquatic invertebrates, such as insect larvae, crustacean and molluscs. Larger specimens readily eat small fish such as perch fry and sticklebacks, and they will enter quite shallow waters in pursuit of them. It has been found that they tend to attack isolated fish at the edge of a shoal, which may be slower than the others or disabled in some way.
In rivers they behave in much the same way as Brown Trout. In still waters, they tend to swim in shoals, often moving at a fast speed and feeding on the surface.