Greenback Cutthroat Trout (Oncorhynchus clarkii stomias): A Technical Conservation Assessment Prepared for the USDA Forest Service, Rocky Mountain Region, Species Conservation Project February 6, 2009 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 Young, M.K. (2009, February 6). Greenback Cutthroat Trout (Oncorhynchus clarkii stomias): a technical conservation assessment. [Online]. USDA Forest Service, Rocky Mountain Region. Available: http://www.fs.fed.us/r2/projects/scp/assessments/greenbackcutthroattrout.pdf [date of access]. ACKNOWLEDGMENTS I thank Claire McGrath, Bruce Rosenlund, and Dave Winters for their comments on an earlier version of this document. The comments of Todd Allison, Warren Colyer, Greg Eaglin, Noah Greenwald, Paula Guenther-Gloss, Christine Hirsch, Jessica Metcalf, Dirk Miller, Kevin Rogers, and Dennis Shiozawa on a companion document on Colorado River cutthroat trout contributed to improving this paper. I appreciate the assistance of Dennis Shiozawa, Brigham Young University; Bill Wengert, Wyoming Game and Fish Department; and Shannon Albeke, Colorado Division of Wildlife, for sharing a number of unpublished reports.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 disturbance on stream ecosystems. COVER ILLUSTRATION CREDIT Illustration of the Greenback Cutthroat Trout (Oncorhynchus clarkii stomias) by © Joseph Tomelleri. 2 3 SUMMARY OF KEY COMPONENTS FOR CONSERVATION OF THE GREENBACK CUTTHROAT TROUT Status Greenback cutthroat trout (Oncorhynchus clarkii stomias) was once presumably distributed throughout the colder waters of the South Platte and Arkansas River basins in Colorado and southeastern Wyoming. Primarily a fluvial species, greenback cutthroat trout may have occupied 10,614 to 13,231 km of streams above 1,800 m in these basins. Nevertheless, the distribution of greenback cutthroat trout has dramatically contracted since the mid-1800’s, particularly from lower-elevation sites. Rebounding from a low of two populations occupying a few kilometers of small streams in 1970, populations regarded as greenback cutthroat trout now occupy up to 72 streams and lakes, 23 of which meet criteria for populations to count toward delisting under the Endangered Species Act. The bulk of these populations are found on federal lands, and many in the South Platte River basin are in Rocky Mountain National Park. Recent genetic evaluations, however, indicate that many populations considered historical remnants of greenback cutthroat trout may be Colorado River cutthroat trout. Individuals of both taxa were often mixed to develop recovery broodstocks, so the number of non-introgressed wild populations of greenback cutthroat trout is uncertain. With the enactment of the Endangered Species Act in 1973, the greenback cutthroat trout was listed as endangered but subsequently downlisted to threatened in 1978. Presently, the U.S. Fish and Wildlife Service is conducting a review to determine whether this designation remains appropriate. Primary Threats Historically, habitat alteration from mining, agriculture, and water development contributed to the extirpation or reduction of large numbers of populations of greenback 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. Ironically, the barriers themselves provide a limitation to dispersal, resulting in most populations of greenback cutthroat trout being restricted to short, headwater stream segments. Lack of connectivity to other populations renders them vulnerable in the short term to extirpation from natural disturbances such as fire, post-fire debris torrents, or floods and in the long term to loss of genetic variability and the potential for evolving in response to changing environmental conditions. This lack of connectivity also contributes to the greatest future threat to the persistence of this subspecies—climate change—because model projections suggest some suitable habitats may shift to higher elevations and precipitation patterns imply there may be large declines in late summer flows. Because these habitat changes are likely to be coupled with greater resource demands (particularly for water) by a rapidly growing human population in the Rocky Mountains, the future for stream populations of greenback cutthroat trout remains uncertain. The prospects for greenback cutthroat trout may be better in the many lakes used for conservation actions, with the understanding that such waters are unlikely to constitute historically occupied habitats. The potential effects of climate change on lake populations are uncertain, but isolation in such environments is likely to alter the genetic structure of some populations. Primary Conservation Elements, Management Implications, and Other Considerations Conservation of greenback 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 of greenback cutthroat trout, but perhaps the most successful long-term strategy will be to eliminate nonnative trout from downstream waters and expand the distribution of greenback cutthroat trout into larger basins exhibiting greater connectivity. Removal of nonnative trout and reintroduction of greenback cutthroat trout into formerly occupied habitats will also reduce the probability of losing all local populations simultaneously as well as offering 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, 2 3 although acknowledging the inherent temporal variability in habitat quality is essential. Given that habitat change is inevitable, managing to permit the continued evolution of this subspecies may be fundamental to its long-term persistence. Systematic conservation planning could prove to be an efficient and successful process for guiding future conservation efforts. 4 5 TABLE OF CONTENTS ACKNOWLEDGMENTS ..............................................................................................................................................2 AUTHOR’S BIOGRAPHY............................................................................................................................................2 COVER ILLUSTRATION CREDIT ..............................................................................................................................2 SUMMARY OF KEY COMPONENTS FOR CONSERVATION OF THE GREENBACK CUTTHROAT TROUT ..3 Status..........................................................................................................................................................................3 Primary Threats..........................................................................................................................................................3 Primary Conservation Elements, Management Implications, and Other Considerations..........................................3 LIST OF TABLE AND FIGURES .................................................................................................................................7 INTRODUCTION ..........................................................................................................................................................8 Goal............................................................................................................................................................................8 Scope..........................................................................................................................................................................8 Treatment of Uncertainty ...........................................................................................................................................8 Application and Interpretation Limits........................................................................................................................9 Publication on the World Wide Web ..........................................................................................................................9 Peer Review ...............................................................................................................................................................9 MANAGEMENT STATUS AND NATURAL HISTORY .............................................................................................9 Management Status ....................................................................................................................................................9 Existing