Case Study: Rainbow Trout

The rainbow trout Oncorhynchus mykiss, provides an example of subtle, yet damaging, changes invasive species can effect on native species. The rainbow trout is native to the Pacific states of the United States. It is disease resistant, produces thousands of eggs at a relatively young age, and adapts well to a hatchery environment, which has made it one of the most popular hatchery fish in the United States, as well as abroad. It has been introduced to countries such as Bolivia, Colombia, Peru, and Belgium to increase fishery capacity and opportunities for sport fishing. After being raised in hatcheries, the trout are often released into streams and rivers to provide prey for sport anglers. This release, however, also provides an opportunity for rainbow trout to impact native fish species and riverine ecosystems. While rainbow trout are often overfished or do not spawn in the wild and must be re-stocked biannually in some places, they still pose a major threat to native fauna.

The rainbow trout can hybridize with other trout species, thereby affecting their genetic integrity. Hybridization with rainbow trout has been a major cause of the decline of native cutthroat trout in Rocky Mountain rivers. Rainbow trout introduced in Lake Tahoe in the 1940s are responsible for the near extinction of Lahontan trout in the area. In addition, the stocking of rivers with hatchery trout has led to an introduction of whirling disease in the open waters of 20 states.

Rainbow trout often consume native fishes and compete with salmonids. Introduced trout eat the endangered humpback chub and the Chiricahua leopard frog. In occupying undercut banks, the trout displace spinedace from preferred habitats, pushing them into open water, which makes them more vulnerable to predation. Rainbow trout also compete with suckers, squawfish, and brook trout, often driving them from preferred feeding territories.

-- Fuller, P.L., L.G. Nico, J.D. Williams, Nonindigenous Fishes Introduced into Inland Waters of the United States (Bethesda, MD: American Fisheries Society, 1999). p. 250-251

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