Native Trout in North America: Impacts of Multiple Stressors

Cutthroat Trout (Oncorhynchus clarkii spp.) were once widespread across western North America and consisted of 14 subspecies.1 However, populations have since been declining due to a variety of threats; leaving only 9 subspecies left in the wild, all of which are under state or Federal protection.2

Cutthroat Trout (Oncorhynchus clarkii)

One of the primary threats to Cutthroat Trout populations is the introduction of nonnative trout. Beginning in the 1800s, nonnative trout introductions became common practice for improving recreational sport fishing in the western United States.3 The ecological ramifications caused by these introductions were not recognized until much later. Brown Trout (Salmo trutta), a native of Europe, have been introduced extensively throughout the Cutthroat Trout’s historical range. Negative effects have been observed in streams with sympatric (i.e. occurring within the same geographical area) populations of Brown and Cutthroat Trout. Due to an overlap in diet and a more aggressive predatory nature, nonnative Brown Trout outcompete native Cutthroat Trout.4 This disadvantage in foraging for food has resulted in a reduction of growth rates and reproductive function.5 Inability to compete in the presence of nonnative fish has also contributed to an upstream shift in the distribution of native trout. In many mountain streams today, the headwaters (i.e. the upstream portion of a river nearest its original source) are occupied by native Cutthroat Trout and the lower portions are dominated by nonnative Brown Trout. This zoning pattern has become useful in conservation practices that implement an “isolation management” strategy, whereby physical barriers are installed to prevent the movement of nonnative trout upstream into the last remaining habitat of native populations.6 Isolating populations in headwater streams away from the negative effects of nonnative Brown Trout is vital in preserving native trout populations. Likewise, preventing contact with nonnative Rainbow Trout (Oncorhynchus mykiss) is also important because of the potential for hybridization and loss of genetic diversity in Cutthroat Trout populations.7

Another growing concern for native Cutthroat Trout populations in western North America is climate change. The amount of snow in the Rocky Mountains is expected to decrease in coming years and with warmer temperatures also forecasted, snowmelt will occur earlier in the springtime thereby altering hydrological patterns in streams and rivers.8 This is problematic for fish that depend on a continuous flow of water throughout the year. Drought has been shown to cause a decrease in the overall abundance and size of Cutthroat Trout populations.9 It is likely that these conditions are a result of a scarcity of food. Trout often feed on aquatic insects that are swept downstream in the moving waters. The number of insects drifting in the water is greatest when flow rates are high, thus under conditions of drought when less water is flowing through a stream the abundance of aquatic insects is rather low.10 Also, when the water level is lower, fish become more concentrated in narrow streams and the effects of competition increases which can have negative impacts on abundance and size.11 Additionally, in response to drought and increased temperatures, Cutthroat Trout have also been shown to move upstream in search of deeper pools and colder water where they can best endure the challenging conditions created by climate change.9 However, this is near impossible when native trout are already living in the upper limits of their range.

Combined effects of nonnative trout and climate change are predicted to extirpate (i.e. the extinction of a population at a local level) roughly 40% of the Colorado River Cutthroat Trout subspecies and leave another 40% vulnerable to extirpation.12 Loss at this scale would be devastating to the species as a whole. Appropriate management is needed to prevent this catastrophe from happening. New introductions of nonnative trout need to be prevented across all regions of North America. Fish barriers need to be repaired or improved to prevent any further migration of nonnative trout upstream. Where possible, nonnatives need to be removed and habitat restored for expanding the current range of native trout. Furthermore, protection of the genetic diversity of native trout is important for allowing populations to have the capacity for adapting to the growing threat of climate change.


  1. Behnke RJ (1988) Phylogeny and classification of Cutthroat Trout. In: Gresswell RE, eds. Status and management of interior stocks of Cutthroat Trout. American Fisheries Society, Symposium 4, Bethesda, Maryland, pp 1-7.
  2. Wilson WD, Turner TF (2009) Phylogenetic analysis of the Pacific Cutthroat Trout (Oncorhynchus clarkii : Salmonidae) based on partial mtDNA ND4 sequences: a closer look at the highly fragmented inland species. Molecular Phylogenetics and Evolution 52: 406-415.
  3. Pister EP (2001) Wilderness Fish Stocking: History and Perspective. Ecosystems 4: 279-286. doi: 10.1007/s10021-001-0010-7
  4. Meredith CS, Budy P, Thiede GP (2015) Predation on native sculpin by exotic brown trout exceeds that by native cutthroat trout within a mountain watershed (Logan, UT, USA). Ecology of Freshwater Fish 24: 133-147. doi: 10.1111/eff.12134
  5. Al-Chokhachy R, Sepulveda AJ (2019) Impacts of Nonnative Brown Trout on Yellowstone Cutthroat Trout in a Tributary Stream. North American Journal of Fisheries Management 39: 17-28. doi: 10.1002/nafm.10244
  6. Kirk MA, Rosswog AN, Ressel KN, Wissinger SA (2018) Evaluating the Trade-Offs between Invasion and Isolation for Native Brook Trout in Pennsylvania Streams. Transactions of the American Fisheries Society 147: 806-817. doi: 10.1002/tafs.10078
  7. McKelvey KS, Young MK, Wilcox TM, Bingham DM, Pilgrim KL, Schwartz MK (2016) Patterns of hybridization among Cutthroat Trout and Rainbow Trout in northern Rocky Mountain streams. Ecology and Evolution 6:688–706.
  8. Stewart IT, Cayan DR, Dettinger MD (2005) Changes toward earlier streamflow timing across western North America. Journal of Climate 18: 1136–1155.
  9. VerWey BJ, Kaylor MJ, Garcia TS, Warren DR (2018) Effects of Severe Drought on Summer Abundance, Growth, and Movement of Cutthroat Trout in a Western Oregon Headwater Stream. Northwestern Naturalist 99(3): 209-221.
  10. Harvey BC, Nakamoto RJ, White JL (2006) Reduced streamflow lowers dry-season growth of Rainbow Trout in a small stream. Transactions of the American Fisheries Society 135: 998–1005.
  11. Uthe P, Al-Chokhachy R, Shepard BB, Zale AV, Kersher JL (2019) Effects of Climate-Related Stream Factors on Patterns of Individual Summer Growth of Cutthroat Trout. Transactions of the American Fisheries Society 148: 21-34. doi: 10.1002/tafs.10106
  12. Roberts JJ, Fausch KD, Hooten MB, Peterson DP (2017) Nonnative Trout Invasions Combined with Climate Change Threaten Persistence of Isolated Cutthroat Trout Populations in the Southern Rocky Mountains. North American Journal of Fisheries Management 37: 314-325. doi: 10.1080/02755947.2016.1264507

*All images are the property of J.Evans

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