Cutthroat Trout Taxonomy
New genetic research led by University of Colorado scientists and published in September of 2012 has produced several fascinating discoveries about Colorado's native cutthroat trout - including the location of the last surviving wild population of the federally protected greenback cutthroat trout.
Researchers analyzed DNA extracted from wild trout and from museum specimens collected from sites around Colorado and New Mexico as far back as 1856, then used that genetic data as a baseline for understanding the current distribution of Colorado's native cutthroat trout.
In addition to identifying the one remaining "true" greenback population, researchers identified two distinct genetic lineages within the range of Colorado River cutthroat trout, one in northwest Colorado and the other centered around the Gunnison River basin. Further research will be needed to understand the relationship between these two lineages.
In all, six lineages of cutthroat appear to have evolved in Colorado, with native ranges that roughly correspond to major drainage basins in the state. The greenback cutthroat trout, Colorado's state fish, was native to the drainages of the South Platte, while the long extinct yellowfin cutthroat trout of Twin Lakes actually appears to have been the native trout of the Arkansas River Basin. Rio Grande cutthroat trout still can be found in streams that drain into the San Luis Valley, while a lineage that used to call the San Juan basin home appears to be extinct as well. More research will be required to resolve whether the remaining two lineages that can be found across Colorado’s western slope are indeed two distinct subspecies – that work is ongoing.
Native ranges of Colorado's six distinct lineages of cutthroat trout follow major drainage basins.
Monitoring Native Cutthroat Trout Populations
Biologists with the Colorado Parks and Wildlife (CPW) 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.
Estimating the size of fish populations in lakes is one of the long-standing challenges facing fish biologists. While CPW 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 CPW 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.