While they do provide a very popular sport fishery in many areas, brown trout have also been effective in outcompeting and displacing native fishes throughout the United States.  Below is an excerpt from a proposal I put together to research the role of brown trout in a native cutthroat trout stream.  While I maintain that brown trout have been very destructive to ecosystems outside of their native range, I don’t necessarily think we should eradicate all brown trout.  Like anything else, they have their place and can provide benefits to areas that otherwise would provide little or no fishery.  It’s up to fisheries managers and the public to make rational decisions about where these fish do and don’t belong. 

(Excerpt)

Biological invasions threaten the biodiversity of native wildlife worldwide (Williamson 1999), with the presence of exotic fishes posing one of the most significant threats to the persistence of native fishes across the United States (Wilcove et al. 1998).  In a review of community response to fish introductions, Ross (1991) notes that 77% of investigations document a decline in native fishes following the introduction of exotics.  In addition, Moyle and Light (1996) present multiple case studies where introductions of exotic species have led to the decline of native fish through interspecific interactions.   Widespread invasion of exotic fishes thus represents a significant problem across the western United States, where native trout species have been imperiled by the introductions of rainbow trout (Oncorhynchus mykiss) (Hitt et al. 2003), brook trout (Salvelinus fontinalis) (Peterson et al. 2004) and brown trout (Salmo trutta) (McHugh and Budy 2006; Wang and White 1994). 

Mechanisms by which introduced species affect native fish include competition, predation, hybridization and disease (Courtenay and Stauffer 1984; Weigel et al. 2003).  While the specific mechanisms are not always well identified, these interactions often lead to the decline and potential extinction of native species (Behnke 1992; Griffith 1988; Quist and Hubert 2005; Ruzycki et al. 2003), highlighting the importance of research in this field.   

One threat of particular concern to cutthroat trout (Oncorhynchus clarkii) in the western United States is the presence of exotic brown trout (McHugh and Budy 2005; McHugh and Budy 2006).   Brown trout are native throughout Europe and have been naturalized through a long history of introductions in over two dozen countries outside of their native range (Lever 1996).  This species was introduced from Germany to the United States in 1883 and stocked in the Pere Marquette River, Michigan, with limited success.  Following further introductions in the 1880’s and 90’s brown trout soon became well established, and have since been distributed throughout the country (Lever 1996; Maccrimmon and Marshall 1968).  Brown trout have been widely viewed as a beneficial introduction in the United States (Courtenay and Stauffer 1984).  This viewpoint may be attributed to the fact that they provide a popular sport fishery and have been considered by many, albeit based purely on speculation, to have little impact on native species.  The widespread introduction and current stocking of brown trout in areas throughout the western United States despite the potential for impacts on native and imperiled fish species (Behnke 1992) provides evidence of this general viewpoint.  As Quist and Hubert (2004) point out, the general public often places higher values on nonnative salmonids than on natives, due to the recreational benefits they provide, which further encourages the introduction of exotic fish species. 

Brown trout were first introduced to waters in Utah circa 1900 (Maccrimmon and Marshall 1968).  Since their introduction to the Logan River, a distinct allopatric distribution between brown trout and cutthroat trout has been identified, apparently due to, in part, the displacement of Bonneville cutthroat trout by brown trout (de la Hoz Franco and Budy 2005).  This distribution is characterized by a dominant brown trout population in lower elevation areas, a dominant cutthroat trout population at higher elevations, and a transition zone where the two species exist in relatively low densities (de la Hoz Franco and Budy 2005).

Bonneville cutthroat trout (Oncorhynchus clarkii utah) (BCT) are native to the Bonneville Basin, which includes Utah and parts of Nevada, Idaho and Wyoming.  While once widespread throughout the Basin, the species currently exists in only about one third of its historical range (Behnke 1992; Duff 1988; Hickman and Duff 1978).  Reasons for this decline include habitat degradation and the negative effects of introduced species (Duff 1988; Young 1995)  However, one of the largest remaining populations of BCT inhabits the Logan River in northern Utah, such that Budy et al. (2006) report BCT densities higher in the Logan River than in any other reported BCT management unit in Utah or Wyoming.  The threats impacting BCT populations rangewide are also an issue for BCT in the Logan river system, and may affect their long term viability. 

Where present together in stream systems, native and introduced fish species often exhibit distinct distribution patterns, or species zones, along stream gradients.  Altitudinal species zonation is a common result of introductions of exotic trout in Western streams (Bozek and Hubert 1992; Rahel and Hubert 1991; Vincent and Miller 1969).  Differences in environmental conditions along the stream gradient are believed to be the underlying factor in determining fish species distribution.  In addition, temperature mediated competition is often used to explain this type of species distribution (DeStaso and Rahel 1994; Taniguchi and Nakano 2000), a theory that one species has a competitive advantage in warm water, while the other is more competitive in cooler water.  Theoretically, these differing advantages work to form distinct species distributions.  

McHugh and Budy (2005) investigated the mechanisms driving trout distribution in the Logan River, particularly temperature mediated competition, and found that adult brown trout routinely outcompeted adult BCT when held together at all elevations and temperature regimes in the Logan.  This suggests that BCT distribution is likely regulated by the presence of brown trout through biotic interactions such as predation and competition.  It also suggests that since brown trout distribution is not limited by competition with BCT, abiotic conditions may regulate the upper limits of brown trout in the Logan River. 

There is no doubt that since their introduction to the Logan River around the turn of the 20^th century, brown trout have been the key factor in displacing native Bonneville cutthroat trout in the river’s lower reaches, and while they provide a popular fishery, the native fish they replaced probably had higher value as both a native and popular sport fish.  

References:

Behnke, R. J. 1992. Native trout of western North America. American Fisheries Society, Bethesda, Maryland.

Bozek, M. A., and W. A. Hubert. 1992. Segregation of resident trout in streams as predicted by three habitat dimensions. Canadian Journal of Fisheries and Aquatic Sciences 70:886-890.

Budy, P., and G. P. Thiede. 2006. Logan River whirling disease study: factors affecting trout population dynamics, abundance, and distribution in the Logan River, Utah. Annual Report to Utah Division of Wildlife Resources.  UTCFWRU:1-72.

Courtenay, W. R., and J. R. Stauffer. 1984. Distribution, Biology and Management of Exotic Fishes. John Hopkins University Press, Baltimore.

de la Hoz Franco, E. A., and P. Budy. 2005. Effects of biotic and abiotic factors on the distribution of trout and salmon along a longitudinal stream gradient. Environmental Biology of Fishes 72:379-391.

DeStaso, J., and F. J. Rahel. 1994. Influence of water temperature on interactions between juvenile Colorado River cutthroat trout and brook trout in a laboratory stream. Transactions of the American Fisheries Society 123:289-297.

Duff, D. A. 1988. Bonneville cutthroat trout: current status and management. American Fisheries Society Symposium 4:121-127.

Griffith, J. S. 1988. Review of competition between cutthroat trout and other salmonids. American Fisheries Society Symposium 4:134-140.

Hickman, T. J., and D. A. Duff. 1978. Current status of cutthroat trout subspecies in the western Bonneville Basin. Great Basin Naturalist 38(2):193-202.

Hitt, N. P., C. A. Frissell, C. C. Muhlfeld, and F. W. Allendorf. 2003. Spread of hybridization between native westslope cutthroat trout, Oncorhynchus clarki lewisi, and nonnative rainbow trout, Oncorhynchus mykiss. Canadian Journal of Fisheries and Aquatic Sciences 60:1440-1451.

Lever, C. 1996. Naturalized fishes of the world. Academic Press, London.

Maccrimmon, H. R., and T. L. Marshall. 1968. World distribution of brown trout, Salmo trutta. Journal of the Fisheries Research Board of Canada 25(12):2527-2548.

Maciolek, J. A., and P. R. Needham. 1952. Ecological effects of winter conditions on trout and trout foods in Convict Creek, California, 1951. Transactions of the American Fisheries Society 81:202-217.

McHugh, P., and P. Budy. 2005. An experimental evaluation of competitive and thermal effects on brown trout (Salmo trutta) and Bonneville cutthroat trout (Oncorhynchus clarkii utah) performance along an altitudinal gradient. Canadian Journal of Fisheries and Aquatic Sciences 62(12):2784-2795.

McHugh, P., and P. Budy. 2006. Experimental effects of nonnative brown trout on the individual- and population-level performance of native Bonneville cutthroat trout. Transactions of the American Fisheries Society 135:1441-1455.

Peterson, D. P., K. D. Fausch, and G. C. White. 2004. Population ecology of an invasion: effects of brook trout on native cutthroat trout. Ecological Applications 14(3):754-772.

Quist, M. C., and W. A. Hubert. 2004. Bioinvasive species and the preservation of cutthroat trout in the western United States: ecological, social, and economic issues. Environmental Science and Policy 7:303-313.

Rahel, F. J., and W. A. Hubert. 1991. Fish assemblages and habitat gradients in a Rocky Mountain - Great Plains stream: biotic zonation and additive patterns of community change. Transactions of the American Fisheries Society 120:319-332.

Ruzycki, J. A., D. A. Beauchamp, and D. L. Yule. 2003. Effects of introduced lake trout on native cutthroat trout in Yellowstone lake. Ecological Applications 13(1):23-37.

Taniguchi, Y., and S. Nakano. 2000. Condition-specific competition: implications for the altitudinal distribution of stream fishes. Ecology 81(7):2027-2039.

Vincent, R. E., and W. H. Miller. 1969. Altitudinal distribution of brown trout and other fishes in a headwater tributary of the South Platte River, Colorado. Ecology 50:464-466.

Wang, L., and R. J. White. 1994. Competition between wild brown trout and hatchery greenback cutthroat trout of largely wild parentage. North American Journal of Fisheries Management 14:475-487.

Weigel, D. E., J. T. Peterson, and P. Spruell. 2003. Introgressive hybridization between native cutthroat trout and introduced rainbow trout. Ecological Applications 13(1):38-50.

Wilcove, D. S., D. Rothstein, J. Dubow, A. Phillips, and E. Losos. 1998. Quantifying threats to imperiled species in the United States. Bioscience 48:607-615.

Williamson, M. 1999. Invasions. Ecography 22(1):5-12.

Young, M. K. 1995. Conservation assessment for inland cutthroat trout, RM-GTR-256, Fort Collins, CO.