Rio Grande Cutthroat Trout

Oncorhynchus clarkii virginalis

The Rio Grande cutthroat trout (RGCT) is native to the Rio Grande, and Pecos and Canadian rivers of Colorado and New Mexico. In Trout and Salmon of North America, Robert Behnke also notes that RGCT may have been native to streams of the Davis Mountains of west Texas, but even if true they have long since been extirpated from that state. Currently, the subspecies is restricted primarily to smaller headwater streams of the Rio Grande drainage in south-central Colorado, and smaller tributaries of the Rio Grande, and Pecos and Canadian rivers in New Mexico. Unlike many other cutthroat trout subspecies, there were no large lake populations and therefore very little if any lacustrine specializations within this Rio Grande subspecies.

Many of the same factors causing declines in the status of inland cutthroat subspecies throughout the West have affected RGCT. Introductions of non-native trout species have been a primary cause for declines. Rainbow trout and non-native cutthroat are especially problematic because of the likelihood of hybridization and subsequent genetic introgression of non-native genes into RGCT populations. The major causes of habitat degradation consist of livestock grazing, timber harvest and associated activities such as road construction, and diversion of water for irrigation. Livestock grazing can remove streamside riparian vegetation and increase erosion of fine sediments into spawning areas. Timber harvest also may increase input of fine sediments into streams and may remove sources of larger wood materials that are needed for building pools and creating complex stream habitat. Roads alter watershed hydrology and may interfere with fish passage if culverts or other stream-road crossings are improperly designed. Adequate supplies of high quality cold water are often in short supply, but especially in the arid Southwest. Irrigation diversions are a concern, more commonly on private lands.

Historic Range Relief Map

Key CSI Findings

• 21% of subwatersheds within the historic range still contain RGCT
• About 10% of historically occupied stream habitat currently contains populations
• Nearly 90% of populations occur in isolated stream segments
• Most currently occupied stream habitats were ranked at "good" or "excellent" condition
• Increasing severity and duration of drought, floods, and wildfire is negatively impacting future population persistence
• Metapopulations and fluvial life histories, both important traits for long term population persistence, are lacking in RGCT

In a 2007 assessment of RGCT, agency and tribal biologists comprising the Rio Grande Cutthroat Trout Conservation Team estimated that a total of 120 "conservation populations" of RGCT remained within the entire historic range. Nearly all populations are isolated with little if any chance of movement among tributaries within drainages. Isolation increases the likelihood of population loss following disturbances such as flood, drought, or wildfire. On the other hand, the isolated nature of these populations may decrease the potential for hybridization or other negative interactions with introduced trouts.

Table of CSI results for 20 CSI indicators within currently occupied and historic range. Range-wide condition and population integrity indicators (first 10 indicators) were scored only for subwatersheds containing existing populations. Remaining habitat and future security indicators were scored with all subwatersheds within the historic range. All indicators are scored from 1 (poorest) to 5 (best): see detailed methods for scoring and rule sets for this subspecies.

Our CSI analysis indicates that the RGCT, like other southwestern trouts, is likely to be severely impacted by climate change. Although it is difficult to attribute any single storm or drought event to climate change alone, most populations of RGCT will be increasingly vulnerable to wildfire, flood, and drought, all factors that are likely to expand in severity and duration during a warming climate. It is likely that the large number of small, fragmented RGCT populations will be particularly vulnerable because they are restricted to short reaches of relatively low stream flow. Drought already has become a major factor in declines of RGCT populations since 2002, including the loss of several small populations. Thus, it is important to understand the 2007 RGCT range-wide assessment results within an existing and future context of rapidly changing environmental conditions.

To improve future conservation status, some populations should be expanded and connected to enhance the likelihood of developing fluvial life histories and interconnected metapopulations. The ability of fish to move within tributaries and larger stream systems increases their chance to survive disturbances and may be especially important during periods of rapidly changing climate. Approximately 25% of subwatersheds within the historic range were at high risk from climate change impacts that include increased summer temperature, increased wildfire, and increased risk of winter flooding. Climate change also may affect interactions between RGCT and introduced fishes in uncertain ways. The best long-term conservation strategy should strive to create a range of isolated to interconnected populations free of introduced salmonids within all major drainages historically occupied by RGCT.

Prepared by Jack E. Williams, TU, 4/16/2008

Conservation Strategies Map

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