Cutthroat Trout

The most diverse trout species in North America, the historical distribution of cutthroat trout covers the broadest range of any stream dwelling trout in the Western Hemisphere. The rugged topography of their range has lead to isolation, which in turn has given rise to fourteen recognized subspecies. Four of these evolved in Colorado: the Colorado River cutthroat trout in drainages west of the continental divide, Greenback cutthroat trout in the South Platte and Arkansas River drainages, and the Rio Grande cutthroat trout in streams that drain into the San Luis Valley. In addition, the yellowfin cutthroat trout was historically found in Twin Lakes at the headwaters of the Arkansas drainage. Unfortunately, this predator that grew to over 10 lbs, is now extinct. All three remaining species have either been petitioned to be listed or are currently listed under the Endangered Species Act. Dramatic reductions in their range have been precipitated primarily by the introduction of nonnative salmonids. Specifically rainbow trout that hybridize with cutthroat trout, and brook and brown trout that tend to replace them in streams and rivers. In an effort to preserve the legacy of these fish, multi-agency conservation teams have been established for each subspecies.

Cutthroat Trout Recovery and Conservation Team documents:

Colorado River cutthroat trout

Greenback Cutthroat Trout Banner

Greenback cutthroat trout

Rio Grande Cutthroat Trout Banner

Rio Grande cutthroat trout

Cutthroat Trout Research

Cutthroat Trout Taxonomy

Molecular techniques have been used recently to distinguish closely related subspecies of organisms that look similar. It was hoped that these methods would clarify relationships between different cutthroat trout subspecies, but instead they have muddied the water, challenging the current paradigm on the heritage of cutthroat trout in the state

Colorado is home to three subspecies of cutthroat trout, all of which are state species of special concern, and have been petitioned to be listed or are currently listed as threatened under the Endangered Species Act. All three subspecies look very similar. As they could not be reliably identified visually or with traditional genetic techniques, their historic range had been used to distinguish them. Recent advances in genetic evaluation methods have finally allowed us to separate the three distinct lineages with assurance. A genetic study published in 2007 confirmed the presence of three discrete cutthroat trout taxa in Colorado, but this work suggested that undocumented stocking activities in the late 1800s may have clouded the heritage of both Colorado River and greenback cutthroat trout. Although the Greenback Recovery Team, which includes the CDOW, had initiated the delisting process for the species after decades of progress, these recent genetic findings have put that effort on hold until we can determine what we actually have, and where. It is critical that we identify their true ancestry before recovery and conservation efforts can continue. Isolation of DNA from museum specimens collected between 1860-1890 is currently being evaluated in hopes of accurately delineating historic ranges of these taxa. Comprehensive genetic assessments of our current populations are also underway to relate current distributions to historic ranges, and thereby infer the influence of historic undocumented stocking on present distribution.

Monitoring Native Cutthroat Trout Populations

Streams

Biologists with the Colorado Division of Wildlife survey many stream cutthroat trout populations every year. These surveys typically are conducted with electrofishing gear on several stream reaches where the habitat appears representative of the remainder of the system. There is increasing concern over how representative the current approach for monitoring cutthroat populations really is, and whether it is the best approach for determining stability and other population characteristics through time. This study explores the variability in population size and structure along a stream course to evaluate the traditional sampling approach, and determine what modifications might improve it.

Lakes

Estimating the size of fish populations in lakes is one of the long-standing challenges facing fish biologists. While Division managers have long used electrofishing to estimate the size of trout populations in streams, those same approaches do not work well in lake environments. The only reliable way to estimate population size in trout lakes at this point is by marking a portion of the population, then recapturing them at a later date with a second survey. Examining the proportion of the population that is marked on this second event, allows estimation of the total population size. That type of mark-recapture project can be very time-consuming and often lacks the precision to provide meaningful results.

As part of the process for delisting greenback cutthroat trout (Colorado’s state fish) under the Federal Endangered Species Act, biologists must demonstrate that a minimum number of fish exist in each lake conservation population as proof of viability. Historically this would have been demonstrated using mark-recapture studies, but in addition to being labor intensive and imprecise, methods used to capture fish for such a study (such as gill nets or traps) might result in injury or mortality to the target species - an unacceptable risk when working with a threatened fish. Unfortunately little work has been done to count trout with hydroacoustic gear because they are so surface oriented, and therefore difficult to locate with a downward pointing beam. Researchers with the Colorado Division of Wildlife and US Fish and Wildlife Service have modified their traditional hydroacoustic system to count trout in high mountain lakes by pointing a narrow beam transducer to the side rather than down so that it the sound beam just skims under the surface where the trout reside. By sweeping the surface of the lake with sound while running parallel transects, counts on returning fish echoes compare favorably with traditional mark-recapture estimates, but are much less labor intensive to generate, and provide much tighter confidence intervals on population size.

Though not trivial, a string of mules can pack this hydroacoustic system and pontoon boat deep into the backcountry where the bulk of these greenback cutthroat trout populations persist. Since most of these high mountain lakes are small, surveys can be repeated in the same day to generate more robust population estimates. With this system, biologists now have a rapid, noninvasive, assessment tool to generate precise estimates of trout population size in high mountain lakes.

Secure native cutthroat trout fishery at Trappers Lake, Colorado

Trappers Lake historically was home to Colorado’s premier wild Colorado River cutthroat trout fishery. Unfortunately, a variety of factors have served to suppress the current population as well as marginalize the value of the fishery due to introgression with Yellowstone cutthroat trout. Recent genetic surveys have revealed that the remaining cutthroat trout are a hybrid swarm. Reduced cutthroat trout numbers have been attributed to a burgeoning brook trout population that is now being thinned by fall trapping efforts. In addition, whirling disease invaded the population in the late 1990s, and has now firmly established itself suppressing recruitment of cutthroat trout further. In an effort to control the spread of the disease, anglers are reminded to decontaminate their gear after fishing at Trappers Lake particularly before heading into the higher elevation lakes such as Little Trappers Lake that remain free of the parasite.

Fortunately, pure eggs taken from Trappers Lake in the 1930s before hybridizing fish were introduced were used to establish at least one population in Colorado and one in the high Sierras of California. By reclaiming several lakes in the Trappers Lake basin, it would be possible to bring the original natives back home. Not only would this help ensure the persistence of the Colorado River cutthroat trout, but it would also allow for the development of an accessible pure broodstock that could be used in future reclamation efforts.

Colorado River Cutthroat Trout

Oncorhynchus clarkii pleuriticus

Colorado River cutthroat trout are one of three subspecies of native trout found in Colorado. They historically occupied portions of the Colorado River drainage in Wyoming, Colorado, Utah, Arizona, and New Mexico. Widespread introductions of non-native salmonids over the last century have served to limit current distributions primarily to isolated headwater streams and lakes. As such, the Colorado River cutthroat trout is designated as a species of special concern in Colorado, and significant resources have been dedicated to conservation of the subspecies. In 1994, member states of the Colorado River Fish and Wildlife Council (a consortium of State Fish and Wildlife agency directors) recognized the need for state wildlife agencies to coordinate conservation actions for Colorado River cutthroat trout, and directed Colorado, Utah, and Wyoming to develop a conservation team. The Conservation Agreement for Colorado River Cutthroat (pdf) is a collaborative effort among state and federal resource agencies designed to provide a framework for the long-term conservation of Colorado River cutthroat trout, and to reduce or eliminate the threats that warrant its status as a species of special concern.

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