Colorado River Cutthroat Trout (Oncorhynchus clarkii pleuriticus): A Technical Conservation Assessment Prepared for the USDA Forest Service, Rocky Mountain Region, Species Conservation Project October 10, 2008 Michael K. Young, Ph.D USDA Forest Service, Rocky Mountain Research Station 800 East Beckwith Avenue Missoula, Montana 59801 Peer Review Administered by American Fisheries Society 1 Young, M.K. (2008, October 10). Colorado River Cutthroat Trout (Oncorhynchus clarkii pleuriticus): a technical conservation assessment. [Online]. USDA Forest Service, Rocky Mountain Region. Available: http://www.fs.fed.us/r2/projects/scp/assessments/coloradorivercutthroattrout.pdf [date of access]. ACKNOWLEDGMENTS I thank Todd Allison, Warren Colyer, Greg Eaglin, Noah Greenwald, Paula Guenther-Gloss, Christine Hirsch, Jessica Metcalf, Dirk Miller, Kevin Rogers, and Dennis Shiozawa for their comments on an earlier version of this manuscript. My thanks also to Claire McGrath, Bruce Rosenlund, and Dave Winters for their reviews of a similar manuscript on a related subspecies. I appreciate the assistance of Dennis Shiozawa, Brigham Young University; Bill Wengert, Wyoming Game and Fish Department; and Dan Brauch, Colorado Division of Wildlife, for sharing a number of unpublished reports. This work was funded by the Species Conservation Project for Region 2 and the Rocky Mountain Research Station, both part of the USDA Forest Service. AUTHOR’S BIOGRAPHY Michael Young has been a Research Fisheries Biologist with the USDA Forest Service Rocky Mountain Research Station since 1989. His work focuses on the ecology and conservation of native coldwater fishes and the effects of natural and anthropogenic disturbances on stream ecosystems. COVER ILLUSTRATION CREDIT Illustration of the Colorado River Cutthroat Trout (Oncorhynchus clarkii pleuriticus) by © Joseph Tomelleri. 2 3 SUMMARY OF KEY COMPONENTS FOR CONSERVATION OF THE COLORADO RIVER CUTTHROAT TROUT Status The Colorado River cutthroat trout (Oncorhynchus clarkii pleuriticus) was once distributed throughout the colder waters of the Colorado River basin above the Grand Canyon, primarily in Colorado, Utah, and Wyoming. Primarily a fluvial species, it is believed to have historically occupied about 34,500 km of streams, of which about 4,850 km are currently occupied by fish thought to be good representatives of this taxon and about 2,900 km are occupied by unhybridized populations or those of particular ecological significance. Somewhat less than half of these populations are found in USDA Forest Service (USFS) Region 2, but all the national forests where Colorado River cutthroat trout historically occurred—Arapaho-Roosevelt, Grand Mesa-Uncompahgre-Gunnison, Medicine Bow-Routt, San Juan, and White River—still support populations. The increased awareness of the status of this subspecies and increased agency conservation efforts since the 1970’s have apparently arrested the rapid loss of known populations and established new populations in some areas. The Colorado River cutthroat trout was first petitioned for listing as threatened or endangered under the Endangered Species Act in December 1999, but in April 2004, the U.S. Fish and Wildlife Service (USFWS) concluded that this subspecies did not warrant listing. In June 2007, the USFWS issued a 12-month finding that this subspecies still did not warrant listing under the Endangered Species Act. Regions 2 and 4 of the USFS have designated the Colorado River cutthroat trout a sensitive species; the Bureau of Land Management has accorded it a similar status; and Colorado, Utah, and Wyoming have given it special management designations. A multi-agency agreement also provides oversight for management of this subspecies. Primary Threats Historically, habitat alteration from mining, agriculture, and water development contributed to the extirpation or reduction of large numbers of populations of Colorado River cutthroat trout, whereas introductions and invasions of nonnative trout probably represent the greatest cause of recent declines and the major impediment to restoration of this fish in much of its historical range. Many populations appear to remain vulnerable to this threat either because barriers to ongoing invasions are absent or because existing barriers may be temporary or have nonnative fish passed over them illegally. A lack of connectivity to other populations increases their vulnerablility to extirpation from “short term” natural disturbances such as fire, post-fire debris torrents, or floods. In the long term the loss of genetic variability and the potential for evolving in response to changing environmental conditions can occur with limited population size. Some climate change model projections suggest suitable habitats may shift to higher elevations and precipitation patterns imply there may be large declines in late summer flows. The lack of connectivity could exasperate this problem by not allowing mobility to suitable habitats. While state and federal agencies along with public organizations are working to reestablish Colorado River cutthroat trout populations, the cumulative effects of past and ongoing impacts coupled with greater resource demands (particularly for water) by a rapidly growing human population in the Rocky Mountains, makes the future for Colorado River cutthroat trout uncertain. Primary Conservation Elements, Management Implications, and Other Considerations Conservation of Colorado River cutthroat trout will require attention to both short-term and long-term threats. In the short term, preventing invasions by nonnative trout through the judicious use of barriers can protect some populations, but perhaps the most successful long-term strategy will be to eliminate nonnative trout from downstream waters and expand the distribution of Colorado River cutthroat trout into larger basins exhibiting greater connectivity. Removal of nonnative trout and reintroduction of Colorado River cutthroat trout into formerly occupied habitats will also reduce the probability of losing all local populations simultaneously as well as offer the potential to conserve the remaining genetic variability represented in extant populations. Habitat improvement may have beneficial effects, particularly if combined with whole-basin changes in activities that degrade habitats; however, acknowledging the inherent temporal variability in habitat quality is essential. Given that habitat change is inevitable, 2 3 managing to permit the continued evolution of this subspecies may be fundamental to its long-term persistence. A strategic, rather than opportunistic, approach to conservation of Colorado River cutthroat trout is likely to be the most efficient and successful. 4 5 TABLE OF CONTENTS ACKNOWLEDGMENTS ..............................................................................................................................................2 AUTHOR’S BIOGRAPHY............................................................................................................................................2 COVER ILLUSTRATION CREDIT ..............................................................................................................................2 SUMMARY OF KEY COMPONENTS FOR CONSERVATION OF THE COLORADO RIVER CUTTHROAT TROUT ...........................................................................................................................................................................3 Status..........................................................................................................................................................................3 Primary Threats..........................................................................................................................................................3 Primary Conservation Elements, Management Implications, and Other Considerations..........................................3 LIST OF TABLES AND FIGURES ...............................................................................................................................7 INTRODUCTION ..........................................................................................................................................................8 Goal............................................................................................................................................................................8 Scope..........................................................................................................................................................................8 Treatment of Uncertainty ...........................................................................................................................................8 Application and Interpretation Limits........................................................................................................................9 Publication on the World Wide Web ........................................................................................................................10 Peer Review .............................................................................................................................................................10 MANAGEMENT STATUS AND NATURAL HISTORY ...........................................................................................10 Management Status ..................................................................................................................................................10