Rio Grande Cutthroat Trout: History, Imperilment, and Management, by Daniel Donahoe

Throughout western North America, 13 colorful subspecies of Cutthroat Trout Oncorhynchus clarki dot the montane landscape. One subspecies of Cutthroat Trout, the Rio Grande Cutthroat Trout Oncorhynchus clarki virginalis, is endemic to southern Colorado and New Mexico (Pritchard et al. 2009). This subspecies of Cutthroat Trout is arguably the gem of the American Southwest, and displays a wide gamut of colors that mimic the picturesque New Mexican sunset.

The shining gem of New Mexico’s streams, the Rio Grande Cutthroat Trout, is a Salmonid (Family Salmonidae) that inhabits some of the state’s coldest and cleanest waters. With most Cutthroat Trout species diverging around 1-2 million years ago, Rio Grande Cutthroat Trout diverged relatively recently. Rio Grande Cutthroat Trout diverged from the nearby Colorado River Cutthroat Trout Oncorhynchus clarki pleuriticus around 100,000 years ago (Pritchard et al. 2009).

This speciation stems from the rugged geography of the Rocky Mountains. For millions of years, the Rocky Mountains geographically isolated populations of Cutthroat Trout throughout western North America. ultimately leading to the evolution of a variety of colorful subspecies (Loxterman and Keeley 2012). Of the 13 subspecies of Cutthroat Trout located across western North America, the Rio Grande Cutthroat Trout is the most-southerly distributed species (Pritchard et al. 2009).

Historic range of the eight major species of Cutthroat Trout (Loxterman and Keeley 2012).

The first reported sighting of Rio Grande Cutthroat Trout was by Francisco Vázquez de Coronado and the Conquistadores in 1541 during their conquest through New Mexico and southwestern North America (Owen 2012). The Rio Grande Cutthroat Trout was once prevalent throughout rivers in Colorado and New Mexico, but currently occupies only 11 percent of its historic range (Shemai et al. 2007). This southerly subspecies of Cutthroat Trout currently inhabitants three river systems within Colorado and New Mexico. The Canadian River, the Pecos River, and the Rio Grande river are the only rivers that currently harbor populations of Rio Grande Cutthroat Trout (Pritchard et al. 2009). With this information, one might beg to ask: how did the Rio Grande Cutthroat Trout become one of the rarest piscine gems of the American Southwest?

One threat to Rio Grande Cutthroat Trout populations came in the form of a foreign competitor; the Brown Trout Salmo trutta. Brown Trout, originally native to cold-water streams of Germany, were introduced in Colorado and New Mexico throughout the late 1880’s with the intent of spurring the states’ angling opportunities (MacCrimmon et al. 1970). These trout are “hardier” than native Rio Grande Cutthroat Trout in that they can tolerate a wider range of temperatures and consume a diverse range of food sources. Rio Grande Cutthroat Trout found in streams where Brown Trout are prevalent often show signs of being outcompeted and exhibit physical damage from aggressive Brown Trout (Shemai et al. 2007).

To make things worse, Rio Grande Cutthroat Trout are fighting another foreign threat: Whirling disease. This disease, originating from Germany, is the work of a prolific parasite Myxobolus cerebralis. This parasite festers inside aquatic Sludge Worms Tubifex tubifex that eventually release “TAM” spores throughout the water column that attach to trout gills (Ayre et al. 2014). This parasite burrows into the gills, working its way into a trout’s spinal cord and feeding on cartilage along the way. This process causes spinal-deformities in infected trout, which leads to erratic behavior in infected trout (Ayre et al. 2014). Trout infected with whirling disease often exhibit tail-chasing swimming behaviors within the water column, making them easy prey for predators (Ayre et al. 2014). If the trout is not preyed upon, it’s dead carcass will sink to the bottom of the river where the parasite’s eggs are released and ingested by Sludge Worms, thus repeating the parasite’s life cycle. This parasite has been credited for substantial decreases in Rio Grande Cutthroat Trout stocks and other trout stocks in Colorado, Utah, Wyoming, and Montana (Ayre et al. 2014).

Aquatic Sludge Worm lifecycle by fishpathogens.net. (Source)

These threats underscore the importance of restoring and preserving intact native Rio Grande Cutthroat Trout populations. To save these populations, fisheries managers must first identify genetically-pure populations of Rio Grande Cutthroat Trout. Genetic sequencing has been conducted on multiple Rio Grande Cutthroat Trout populations from a variety of streams and drainages around New Mexico and Colorado (Pritchard et al. 2009). Even though genetic sequencing is becoming a part of normal, everyday fisheries management, researchers will find it difficult to pinpoint a genetically pure population of Rio Grande Cutthroat Trout, as these populations have developed small, but significant differences in their genomes. Populations of Rio Grande Cutthroat Trout that are separated by only a few kilometers have been found to show distinct differences in their genomes (Pritchard et al. 2009).  Historic stockings of genetically dissimilar Rio Grande Cutthroat Trout have diluted the gene pool, effectively creating mongrel Rio Grande Cutthroat Trout. This micro-geographic structuring of Rio Grande Cutthroat Trout populations across the New Mexican country-side has made managing Rio Grande Cutthroat Trout difficult. Fisheries managers must tweak their management strategies depending on the genetic composition of a single population of Rio Grande Cutthroat Trout contained within a single sub-drainage (Pritchard et al. 2009).

Rio Grande Cutthroat Trout by loeflyfishing.com. (Source)

           

Current efforts to conserve Rio Grande Cutthroat Trout are being carried out by a variety of state and federal conservation agencies. One agency, the U.S. Forest Service, has advised the construction of natural and artificial dams to impede the spread of nonnative trout species that can outcompete native Rio Grande Cutthroat Trout (Young 1995). Along with dams, electroshocking has been used to eradicate nonnative trout from pools to eliminate the chance of competition on Rio Grande Cutthroat Trout (Young 1995). Efforts to manually remove invasive trout have also been instituted by conservation agencies. Federal and state conservation agencies within Colorado and New Mexico require fishermen to dispose of nonnative trout species if they are caught while fishing in streams that also harbor Rio Grande Cutthroat Trout (Quist and Hubert 2004). Additional policies have been instituted such as creel limits and protection of Rio Grande Cutthroat Trout spawning areas. Fortunately, these policies have been mildly successful in allowing Cutthroat Trout return to its native range (Quist and Hubert 2004).

    

            The Rio Grande Cutthroat Trout has proven to be an integral part of the American Southwest. This trout, displaying a wide gamut of colors that mirror the picturesque New Mexican sunset, has received a variety of threats from foreign invaders. To counteract these threats, federal and state conservation agencies have instituted the use of dams, creel limits, and other methods to eradicate competitive nonnative trout. These efforts help conserve the Rio Grande Cutthroat Trout for future generations of Coloradans and New Mexicans to enjoy.

References

Ayre, K. K., C. A. Caldwell, J. Stinson, and W. G. Landis. 2014. Analysis of Regional Scale Risk of Whirling Disease in Populations of Colorado and Rio Grande Cutthroat Trout Using a Bayesian Belief Network  Model. Risk Analysis: An International Journal 34(9):1589–1605.

Loxterman, J. L., and E. R. Keeley. 2012. Watershed boundaries and geographic isolation: patterns of diversification in cutthroat trout from western North America. Evolutionary Biology 12:38.

MacCrimmon, H. R., T. L. Marshall, and B. L. Gots. 1970. World Distribution of Brown Trout, Salmo trutta: Further Observations. Journal of the Fisheries Research Board of Canada 27(4):811–818.

Owen, J. 2012. Trout. Reaktion Books, United Kingdom.

Pritchard, V. L., J. L. Metcalf, K. Jones, A. P. Martin, and D. E. Cowley. 2009. Population structure and genetic management of Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis). Conservation Genetics 10(5):1209.

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 &  Policy 7(4):303–313.

Shemai, B., R. Sallenave, and D. E. Cowley. 2007. Competition between Hatchery-Raised Rio Grande Cutthroat Trout and Wild Brown Trout. North American Journal of Fisheries Management 27(1):315–325.

Young, M. K. 1995. Conservation assessment for inland cutthroat trout. United States Forest Service General Technical Report RM-256.