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Rainbow trout are unusual in that although can spend their entire life in fresh water, they are capable of migrating to sea water when conditions are right, though they must return to fresh water to breed.
The freshwater form is usually called “rainbow trout” or “redband trout”, while the marine form is often called “steelhead”, but these populations belong to the same species.
Rainbow trout are the smaller variety, found only in fresh water.Rainbow trout spend their adult lives in the ocean, but return to spawn in the streams in which they were born. They occur in cool streams up to 4,500 m in elevation.
Rainbows have small black spots along their back, dorsal fin and caudal fin. Rainbows have a pink streak that runs from the gill cover to the caudal fin. The color of a rainbow’s back varies from blue or green to a yellow-green or brown. Rainbows are distinguished from their cousins, the cutthroat trout (Oncorhynchus clarki) by their short maxilla, which reaches to, but not past the rear margin of the eye. Rainbow trout also lack hyoid teeth, a feature present in cutthroat trout.
Rainbow trout range from 30 to 60cm in length and in weight from 3 kg to 4 kg.