Rainbow Trout (Oncorhynchus mykiss) are native to only a small portion of Montana, specifically the drainages that flow toward the Pacific Ocean. However, the vast, thriving populations found in famous rivers today are not indigenous. Their widespread presence is a direct result of decades of extensive stocking programs. This history means the fish is simultaneously a rare native species and a widespread, non-native game fish, influencing modern fisheries management.
The Limited Native Range
The historical distribution of Rainbow Trout in Montana was naturally restricted by the Continental Divide. Only populations in drainages on the western side of this divide, which flow into the Pacific Ocean via the Columbia River system, are considered native to the state. This indigenous range is primarily confined to the upper Kootenai River drainage in the northwestern corner of Montana, where a small population of native Redband Trout, a distinct subspecies of O. mykiss, is recognized.
This genetically distinct population is classified as a Fish of Special Concern due to its limited scope and the threats posed by non-native strains. The native trout historically occurred across northwestern Montana, including parts of the Clark Fork River drainage. These original populations developed specific local adaptations over millennia, giving them unique genetic markers suited to their particular environment. Their existence forms the biological basis for the claim that Rainbow Trout are native, despite their geographically limited and threatened capacity.
The History of Widespread Stocking
The widespread distribution of Rainbow Trout across Montana, especially east of the Continental Divide where they are not native, began in the late 19th century. Stocking efforts commenced around 1889, introducing the species into virtually every suitable body of water, including rivers that previously held only native species like Westslope Cutthroat Trout and Arctic Grayling. The motivation was to enhance angling opportunities by introducing a fish that was hardier, easier to cultivate in hatcheries, and considered a sportier catch than native trout.
The stocking utilized numerous non-native hatchery strains, often originating from California or the Great Lakes region. Over the 20th century, hundreds of millions of Rainbow Trout were introduced into Montana’s rivers and lakes. This intensive management established self-sustaining, or “wild,” populations of non-native Rainbow Trout in major systems like the Madison, Big Hole, and Yellowstone Rivers. Recognizing the detrimental effects, the state began halting the stocking of trout in streams that already supported wild trout in 1974, shifting focus to wild fish management.
Ecological Consequences of Introduction
The introduction and proliferation of non-native Rainbow Trout strains created significant ecological challenges for Montana’s true native trout, particularly the Westslope and Yellowstone Cutthroat Trout. The most detrimental consequence is introgressive hybridization, the interbreeding between introduced Rainbow Trout and native cutthroat species, which produces a hybrid commonly called a “Cutbow.” This crossbreeding is a conservation concern because it reverses evolutionary isolation and leads to the loss of the native species’ genetic integrity.
As non-native Rainbow Trout genes spread through native populations, the cutthroat trout lose their locally adapted traits, making them less resilient to environmental changes. This genetic dilution is a leading threat to the remaining genetically pure populations of native cutthroat trout, which now occupy only a fraction of their historical range. Furthermore, introduced Rainbow Trout compete with native species for limited resources such as food and spawning habitat. Fisheries managers now view the widespread non-native Rainbow Trout as a management challenge, focusing conservation strategies on reducing hybridization and competition to protect native genetic strongholds.