The Rainbow Trout (Oncorhynchus mykiss), also called steelhead trout, is a single species of trout native to the Pacific Ocean and in North American rivers and lakes west of the Rocky Mountains. Rainbow Trout are the smaller variety, found only in freshwater. Steelhead spend their adult lives in the ocean, but return to spawn in the streams in which they were hatched. Rainbow trout have been introduced to at least 45 countries, and every continent except Antarctica. They occur in cold, cool streams up to 4500m in elevation.
Rainbows and steelhead 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. Steelhead usually lack the pink stripe, except when young or spawning, and have chrome-colored sides. Rainbows are distinguished from their cousins, the cutthroat trout, 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.
Rainbows range from 30 to 45 cm (12 to 18 inches) in length. Steelheads grow longer, ranging from 50 to 100cm (20 to 40 inches) in length. Steelhead range in weight from 2.5 kg to 10 kg.
Like salmon, steelhead are anadromous: they return to their original hatching ground to spawn. Different populations of steelheads migrate upriver at different times of the year. Summer-run steelhead migrate between May and October, before their reproductive organs are fully mature. They mature in freshwater before spawning in the spring. Winter-run steelhead mature fully in the ocean before migrating, between November and April, and spawn shortly after returning. Unlike salmon, steelhead are iteroparous and may make several spawning trips between fresh and salt water.
Redband trout, sometimes called "Redsides," are another subspecies of Rainbow Trout and are typically found further inland than regular Rainbow Trout.