The Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis) is the southern-most subspecies of cutthroat trout in the Western United States. Historically, the subspecies occupied over 10,000 km of streams in the Rio Grande, Pecos, and Canadian River basins of Colorado and New Mexico. Introductions of non-native fishes and habitat loss have reduced this subspecies to 120 conservation populations occupying less than 12 percent of its historical range (1,500 km). In 2008, the subspecies was added to the Endangered Species Act Candidate List. One of the major threats to the subspecies is the threat of climate change. To help resolve some of the uncertainty related to climate vulnerability, the New Mexico Cooperative Fish and Wildlife Research Unit, New Mexico State University and US Geological Survey with the Crustal Geophysics Laboratory in Denver initiated a research effort in 2008 to assess the threat of increasing stream temperatures on the remaining Rio Grande cutthroat populations.

Research revealed the upper thermal limits of Rio Grande cutthroat trout is lower than non-native trout that have been introduced into their historic habitat such as rainbow trout and brown trout. This spurred the development of a long term monitoring program to assess both current stream temperature and hydrology (stream flow) throughout the remaining populations. Since then, temperature monitoring has revealed the majority of these populations occupy thermally suitable conditions, however, nearly 70 percent of these populations occupy very small streams with summer baseflow less than 1.0 cubic feet per second with several streams becoming dry.

Drought conditions throughout the southwestern U.S. in 2002 extirpated 14 Rio Grande cutthroat trout populations. Nearly ten years later (2011-2012), the southwest experienced the worst drought on record with 'exceptional' drought conditions lasting well over 350 days. These observations resulted in the development of a novel data logger that could measure both temperature and intermittency of streams. Results revealed that small streams with very low summer baseflow (less than 0.50 cfs) had become intermittent with large portions becoming dry. Just as important, these streams rewetted and were flowing at the time of annual sample collections.

While the majority of Rio Grande cutthroat trout experience optimum stream temperatures, these populations are in high elevations that relegate them to small headwater streams of very low stream flow. Future increases in air temperature and decreases in precipitation will increase aridity across the subspecies' current range, increasing the duration and severity of droughts. These populations may be at risk of further loss of habitat as nonnatives are predicted to move upstream to avoid increasing stream temperatures. The team plans to continue the monitoring program to track future changes in both increasing stream temperatures and decreases in stream flow and relate these changes to the persistence of Rio Grande cutthroat trout.

Each month, the ONB features articles from Cooperative Fish and Wildlife Research Units across the country. Working with key cooperators, including WMI, Units are leading exciting, new fish and wildlife research projects that we believe our readers will appreciate reading about.