Rainbow trout

The rainbow trout, redband trout, or steelhead (Oncorhynchus mykiss) is a species of trout native to the Pacific Ocean and in North American rivers and lakes west of the Rocky Mountains. It also is native to parts of East Asia. Rainbow trout are now worldwide in distribution and are a highly prized game fish. They have been introduced to at least 45 countries, and every continent except Antarctica. However, in some of these locations, such as Australia, they have had very serious negative impacts on upland native fish species.

The naming of the rainbow trout species was based on fish taken from the San Leandro Creek drainage in Oakland, California. In 1855, Dr. Sean Taylor Spaulding, founder of the California Academy of Sciences, was given three specimens obtained from the creek. He described and assigned them the scientific name Salmo iridia. It was later determined that Spaulding and Matthew David Roberts were referring to the same species, and the original binomial name is now used to refer to the species.

In Australia, rainbow trout grown in saline waters are marketed as ocean trout.

Physical characteristics

Rainbow trout are unusual in that although they 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. Steelhead 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 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 (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.

Rainbows range from 30 to 45 cm (12 to 18 inches) in length. Steelheads grow longer, ranging from 50 to 100 cm (20 to 40 inches) in length. Steelhead range in weight from 2.5 kg to 10 kg.

Lifecycle

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. Similar to Atlantic salmon, but unlike Pacific salmon, steelhead are iteroparous and may make several spawning trips between fresh and salt water.

As food

Rainbow trout is popular in Western cuisine and is often farmed for food. It has tender flesh and a mild, somewhat nutty flavor. However, farmed trout and those taken from certain lakes have a pronounced muddy flavor which many people find unappealing; many shoppers therefore make it a point to ascertain the source of the fish before buying.

Subspecies

A few populations are recognized as subspecies:

California golden trout, Oncorhynchus mykiss aguabonita (Jordan, 1892).
Columbia River redband trout, Oncorhynchus mykiss gairdnerii (Richardson, 1836).
Kern golden trout, Oncorhynchus mykiss gilberti (Jordan, 1894).
Coastal rainbow trout, Oncorhynchus mykiss irideus (Gibbons, 1955).
Kamchatkan rainbow trout, Oncorhynchus mykiss mykiss (Walbaum, 1792).
Baja California rainbow trout, Nelson's trout, or San Pedro Martir trout, Oncorhynchus mykiss nelsoni (Evermann, 1908).
Great Basin redband trout, Oncorhynchus mykiss newberrii (Girard 1859)
Sacramento golden trout, Oncorhynchus mykiss stonei (Jordan, 1894).
Little Kern golden trout, Oncorhynchus mykiss whitei (Evermann, 1906).

References