Animal Field Guide

Rainbow Trout

Oncorhynchus mykiss
(Salmonidae)

Global Rank: G5
State Rank: S5

Agency Status
USFWS:
USFS: none
BLM: none

The rainbow trout is Montana's number one game fish. Rainbow trout were introduced from numerous hatchery stocks into virtually every suitable habitat in the state, beginning in 1889. Scientists believe that only the rainbow trout of the upper Kootenai River drainage are native to this state. This small group of natives are a Fish of Special Concern. Rainbow trout introductions have caused a severe reduction in the range of the native cutthroat trout through hybridization and competition. Rainbow trout fare well under a wide range of habitat conditions from ponds to reservoirs, lakes, and streams. Various strains of rainbow trout, like breeds of cattle, are used for different purposes. The state has stocked hundreds of millions of rainbow in state waters in the last 100 years. However, today the policies have evolved and rainbow trout are stocked primarily in lakes and reservoirs but no longer in streams. The state record for rainbow trout is over 33 pounds, and fish of up to 10 pounds are common in some of our most productive waters. Rainbow are efficient at feeding on plankton, aquatic and terrestrial insects, and occasionally some smaller fishes. They spawn in early spring in running water, usually April or May, and maintain themselves quite nicely if the habitat is not degraded.

Montana: both pure and moderately hybridized populations of westslope cutthroat trout have a high incidence of basibranchial teeth, whereas pure rainbow trout lack these teeth; presence of basibranchial teeth in some individuals of a rainbow trout population indicates hybridization with westslope cutthroat trout (leary et al. 1996). Individuals living in large or turbid lakes or under ice tend to be iridescent silver.

Important game fish. Various populations have been cultured and introduced due to unique qualities (e.g., large lake form Kamloops; Eagle Lake rainbow, adapted to alkaline waters and usually piscivorous; Arlee strain, noted for fast growth, disease resistance, and high catchability (Sublette et al. 1990). Used in carcinogen testing (Metcalfe 1989).

Migrates into tributary streams from lakes to spawn and river populations may also move upstream and/or into tributary streams.

Cool clean streams, lakes, res., farm ponds. Able to withstand wider range of temperatures than most trout. Spawns in streams over gravel beds (Brown 1971, Holton 1981).

Feed mainly on aquatic insects but eat what is available to them. Large adults also eat fish (Brown 1971). River populations mostly insect eaters while zooplankton and forage fish are important in Lake Koocanusa (Huston et al. 1984).

Populations of native and introduced rainbow trout as well as their hybrids coexist in Kootenai River drainage.

Sexually mature 2-3 years but sometimes 1 year. Spawns April-July depending on water temperatures. Eggs hatch in 50 days at 50 degrees F. (Brown 1971). Young smolt at 0-2 years in tributaries and lake Koocanusa (Huston et al. 1984, May and Huston 1979).