Introduction

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Cutthroat Trout (Oncorhynchus clarkii) are a broadly distributed native species in the western United States that have evolved into twelve subspecies. The Rio Grande Cutthroat Trout (Oncorhynchus clarkii virginalis, hereafter ‘RGCT’) is the southernmost Cutthroat Trout subspecies, and is found from southern Colorado to central New Mexico. This species occupies isolated high-elevation stream segments in the Rio Grande, Canadian, and Pecos River basins. Declines in RGCT throughout their range, candidacy for listing under the Endangered Species Act, and an interest in maintaining their status as a sport species in New Mexico have made them a management priority. 

Like many native salmonids across the western United States, drivers of RGCT decline include invasions by non-native salmonids, habitat loss, and fragmentation. Non-natives of concern include Brown Trout (Salmo trutta) and Brook Trout (Salvelinus fontinalis), among others. RGCT may continue to be displaced through the interaction of climate change climate change
Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.

Learn more about climate change and non-native species invasions, exacerbating the risk of extinction. Managers are therefore interested in identifying the greatest risks and most effective conservation strategies for RGCT, as well as novel techniques to manage threatening non-natives. 

Researchers and managers at New Mexico State University, Colorado State University, New Mexico Department of Game and Fish, Colorado Parks and Wildlife, and U.S. Geological Survey (USGS) are researching the impacts of climate change and non-native species to inform management strategies for RGCT in New Mexico and Colorado. 

Key Issues Addressed

RGCT have suffered range constrictions as a result of habitat alteration and competitive exclusion by non-native species. RGCT now occupy an estimated 12% of their historical distribution, and around 55% of their historical distribution is occupied by non-native salmonids, primarily Brown Trout. Similarly, non-native Brook Trout are considered an increasing threat to RGCT and other native salmonids in the western United States. Hybridization between RGCT and non-native salmonids is a key concern for managers. 

While threats posed by non-native species throughout the range of RGCT have been increasingly recognized, future impacts of climate change on RGCT populations remain somewhat unclear. A lack of experimental data on RGCT thermal tolerances (how much heat stress individuals can survive) and uncertainties regarding changes in stream temperature and the timing of seasonal events in streams occupied by RGCT have confounded the ability to predict potential effects of climate change to exacerbate population declines. 

Management strategies frequently aim to prevent the expansion and reduce or eradicate populations of non-native species through chemical or mechanical removal and the construction of fish barriers. Because of the high resource requirements of these methods, managers are interested in assessing the relative influence of threats posed by non-native species compared to other threats like climate change to prioritize conservation efforts. For the same reason, managers are interested in new, improved, and more efficient strategies for managing non-native species. 

Project Goals

• Use climate models, assessments of thermal stress, and stream temperature measurements to predict future threats to RGCT due to climate change
• Use statistical methods to determine the comparative impacts of threats to RGCT populations caused by genetic risks (including hybridization), population demographics (including habitat suitability), and abiotic factors (stream drying and wildfire), as well as stream temperature and base flows
• Develop new non-native species management strategies

Project Highlights

Public-Private Partnerships: State and Federal agencies across Colorado and New Mexico partnered with Turner Enterprises, Inc. to restore 200 km of stream habitat in the Rio Costilla Watershed for RGCT.

• Thermal Tolerance Won’t Cushion Cutthroat: Researchers expected RGCT to be better able to tolerate warm stream temperatures than co-occurring non-native salmonids because of their southern range. In contrast, researchers found that RGCT have a lower tolerance to high temperatures than co-occurring non-natives. This lack of “thermal cushion” means that RGCT are unlikely to outcompete co-occurring non-natives under warmer conditions.
• Increasing Nightly Temperatures: Researchers used continuous temperature loggers to monitor stream temperature, which showed increased low temperatures at night. Typically, nightly low temperatures allow RGCT to recover from high thermal stresses during the day. As nightly lows warm, RGCT may be more susceptible to thermal stress. These shifting temperatures may additionally lead to phenological mismatch, in which life history events triggered by temperature changes are mistimed, with potential impacts to both survival and reproduction.
• Non-Natives Pose the Greatest Threat: In statistical models, 60% of the 78 conservation populations that were invaded by non-natives or lacking a complete fish barrier were predicted to be extirpated by 2080. The remaining invaded populations averaged only a 10% chance of survival. Models indicate that populations separated by a fish barrier are eight times more likely to survive in the long term than those without. Rising stream temperatures, however, were predicted to threaten only 9% of populations.
• Trojan Trout: Researchers established a broodstock broodstock
  The reproductively mature adults in a population that breed (or spawn) and produce more individuals (offspring or progeny).
  
  Learn more about broodstock of YY “supermale” Brook Trout (phenotypic males that have two Y chromosomes, rather than the typical X and Y chromosomes) using estradiol-infused feed to induce sex reversal. The initial hatchery broodstock development took four years and cost around $10,000. The supermales have successfully shifted the sex ratio of some wild Brook Trout populations within four spawning seasons. This strategy may enable managers to effectively reduce threats to RGCT posed by non-native salmonids by shifting the sex ratio of the target population to all males, preventing further reproduction, and ultimately casuing the population to collapse.

Lessons Learned

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While warming trends are likely to reduce suitable habitat occupied by RGCT, they may also cause previously suboptimal habitat to become habitable. This is especially true for populations that occur at the upper extremities of the current range, where reproduction is limited by low summer temperatures. For these populations, reproduction rates are likely to increase with warming stream temperatures. Managers can take advantage of newly available habitat through targeted releases. 

Sensitivity analyses indicate that fish barriers and non-native control have the greatest conservation potential for RGCT in the long term. Future reintroduction efforts should be focused in watersheds with intact stream habitat where populations can be maintained above natural or manmade barriers. Construction of one fish barrier typically costs around $400,000. Chemical removal methods have been successful in removing Brown Trout in some reaches of RGCT habitat, including Rio Grande headwaters. However, non-natives remain a problem in areas where piscicide use is not appropriate given its non-selective effects. Mechanical removal is costly and not efficient at removing all non-native fishes. 

In developing the statistical model for threat assessment, involving a diverse stakeholder group was key to identifying the most important threats and their perceived intensity. Researchers were surprised to find that increasing stream temperature was not a major threat to the persistence of most RGCT populations. The statistical model was useful in identifying research gaps, particularly where some threats may have been underestimated due to a lack of available data. 

In contrast to conventional stream temperature measurements which only provide insights at specific time points, continuous temperature loggers allowed researchers to assess instantaneous changes in streams occupied by RGCT, particularly as they relate to seasonal events like snowmelt. Temperature logger data collection was facilitated by citizen science initiatives, enabling researchers to overcome labor challenges which would otherwise limit its use.

Next Steps

• Prevent non-native fishes from moving into newly available habitat by blocking their passage with fish barriers and removing them from new habitat when found
• Construct quantitative food webs that enable researchers to directly assess what fish are consuming, and draw comparisons with what food resources are available in their environment: For RGCT, constructing a quantitative food web will provide insight into how competition may adversely impact RGCT. Preliminary results suggest that co-occurring non-native salmonids outcompete RGCT for high value food resources.
• Improve existing habitat conditions for RGCT, which require a minimum length of habitable stream to survive: State, federal, and private institutions have made efforts to improve habitat conditions for RGCT in areas of high conservation potential. Habitat improvements seek to reduce riparian riparian
  Definition of riparian habitat or riparian areas.
  
  Learn more about riparian zone degradation from grazing and fire, increase tree cover, shade, and undercut banks, increase stream woody debris, and increase pool depth and structure structure
  Something temporarily or permanently constructed, built, or placed; and constructed of natural or manufactured parts including, but not limited to, a building, shed, cabin, porch, bridge, walkway, stair steps, sign, landing, platform, dock, rack, fence, telecommunication device, antennae, fish cleaning table, satellite dish/mount, or well head.
  
  Learn more about structure . Increases in protections for riparian areas are ongoing.
• Remove non-native species and construct fish barriers throughout the RGCT range: Through the Rio Costilla joint venture initiative, New Mexico Game and Fish, U.S. Forest Service, Tribes, and non-governmental organizations like Turner Enterprise, Inc. and Trout Unlimited are working to remove and prevent the movement of non-native salmonids from large sections of the Rio Grande Basin in order to improve conditions for RGCT.

Funding Partners

• USGS National Climate Adaptation Science Center
• USGS South Central Climate Adaptation Science Center
• U.S. Fish and Wildlife Service, Region II, Federal Aid Grant F-243
• Turner Enterprises Inc.
• Mesilla Valley Audubon Society
• Western Division of the American Fisheries Society Eugene Maughan Graduate Student Scholarship and small project and travel grants

Resources

• Rio Grande Cutthroat Trout Conservation Strategy
• Rio Grande Cutthroat Trout Status Assessment
• Zeigler, M. P., Rogers, K. B., Roberts, J. J., Todd, A. S., and Fausch, K. D. (2019). “Predicting persistence of Rio Grande cutthroat trout populations in an uncertain future.” North American Journal of Fisheries Management 39(5): 819-848.
• Zeigler, M. P., Brinkman, S. F., Caldwell, C. A., Todd, A. S., Recsetar, M. S., and Bonar, S. A. (2013). “Upper thermal tolerances of Rio Grande cutthroat trout under constant and fluctuating temperatures”. Transactions of the American Fisheries Society 142(5): 1395-1405.
• Schill, D. J., Heindel, J. A., Campbell, M. R., Meyer, K. A., & Mamer, E. R. (2016). “Production of a YY male brook trout broodstock for potential eradication of undesired brook trout populations.” North American Journal of Aquaculture 78(1): 72-83.
• ScienceBase Catalog (2013). The effects of drought on Rio Grande cutthroat trout: The role of stream intermittency.
• ScienceBase Catalog (2018). Susceptibility of Rio Grande cutthroat trout to displacement by Non-native Brown Trout and implications for future management. 

Contacts

• Colleen Caldwell, Affiliate Professor and Unit Leader, USGS New Mexico Cooperative Fish and Wildlife Research Unit: ccaldwel@usgs.gov
• Matthew Zeigler, Colorado River Basin Native Fish Supervisor, New Mexico Department of Game and Fish: matthew.zeigler@state.nm.us
• Bryan Bakevich, Rio Grande Basin Native Fish Supervisor, New Mexico Department of Game and Fish: bryan.bakevich@state.nm.us

CART Lead Author

• Nicolas Katz, CART Student Intern, University of Arizona

Suggested Citation

Katz, Nicolas A. (2022). “Threat Assessment and Conservation Efforts for Rio Grande Cutthroat Trout” CART. Retrieved from https://www.fws.gov/project/conservation-rio-grande-cutthroat-trout.