Montana Water Center - aquatic sciences lab

Aquatic Sciences Laboratory - Species Profiles

Rainbow Trout
Oncorhynchus mykiss

Rainbow trout were introduced in Montana (except for the native redband rainbow trout, Oncorhynchus mykiss gairdneri). Reportedly, the first rainbows were stocked into the headwaters of the Madison River in 1889, and have been introduced and naturally dispersed throughout most of the state since. Historically, most hatchery rainbow trout were derived from steelhead (anadromous subspecies) or redband rainbow trouts (residential subspecies) from California, with propagation beginning in the late 19th century. The U.S. Fish Commission in the late 19th and early 20th century began hatchery production of a mixture of steelhead and redbands from the McCloud River in California (however, other strains were later added from northern California and Oregon populations). These propagated fish were then shipped to various states and foreign countries, but since that time that original propagated strain has branched into various “hatchery” and “wild” strains due to natural and hatchery selection.

All wild rainbow trout populations spawn when water temperatures begin to rise, commonly this is the spring, but there are abnormal examples. Generally, the cue temperature is 42-44 ºF (6-7 ºC). Some domesticated strains have been selectively bred to spawn in the fall for aquaculture production and stocking reasons. Generally, no introduced rainbow trout make the saltwater migration as displayed in steelhead, but many populations still undergo minor migratory runs within their waters. Additionally, some populations, such as those in the Great Lakes, show many characteristics of their steelhead ancestors. Rainbow trout have many strains with varying characteristics in consideration of everything from temperature tolerance to disease resistance. They are similar to most salmonids because females build redds for spawning in gravel and, generally, a single male accompanies her. However, “sneaker” males do occur in rainbow trout. At 50 ºF, most eggs hatch within 50 days.

Rainbow trout are the most propagated trout species for both stocking and commercial purposes. Due to their popularity as a sportfish and aquaculture species, they have been spread across the United States and much of the world (Africa, Asia, Europe, North America and South America). Introduced rainbow trout pose a problem to native trout through competition and hybridization, as well as by pressuring other parts of the ecosystem (similar to other invasive species). Consider the case of mountain yellow-legged frog populations that declined in Sixty Lake Basin, Kings Canyon National Park, California due to the stocking of rainbow and brook trout into historically fishless lakes. Hybridization is a difficult problem to diagnose since a small portion of the gene pool may produce no outward signs of genetic variation. Moves to limit the detrimental effects of rainbow trout are frustrated by their value to fisherman and the economy, illegal stocking, and misunderstood reclamation plans, among other things. Currently, the greatest threat to rainbow trout in the Rocky Mountain west is the introduced parasite Myxobolus cerebralis, the cause of whirling disease.

References:

Brown, C.J.D. 1971. Fishes of Montana. Big Sky Books, Bozeman, MT.
Behnke, R.J. 1992. Native Trout of Western North America. American Fisheries
Society Monograph 6.
Behnke, R.J. 2002. Trout and Salmon of North America. The Free Press, New York.
Food And Agriculture Organization of the United Nations: Fisheries Global Information
System http://www.fao.org/figis
Schade, C.B. & Bonar, S.A. 2005. Distribution and abundance of nonnative fishes in
streams of the western United States.
Vredenburg, V.T. 2004. Reversing introduced species effects: Experimental removal of
introduced fish leads to rapid recovery of a declining frog. PNAS 101: 7646-7650.

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