Greenback Cutthroat Trout

Oncorhynchus clarkii stomias

Greenback cutthroat trout (GCT) are the most easterly of all cutthroats, evolving from ancestral steelhead and coastal cutthroat trout that migrated up with Columbia and Snake River drainages to the Yellowstone and Green/Colorado systems. Robert Behnke believes that the Colorado River cutthroat, which likely invaded the South Platte basin by headwater stream capture, are the closest relatives to the GCT.

Greenback cutthroat trout (GCT) once were abundant in the South Platte and Arkansas river drainages in central Colorado and a small portion of adjacent Wyoming. By the early 1900s, however, GCT had been extirpated from most of this native range because of habitat destruction and the introduction of non-native trouts. GCT appear to be perhaps the most susceptible native trout to displacement by non-native trout introductions.

Genetically pure greenbacks were thought to be extinct by the 1930s due to over harvest and introductions of non-native fishes. Nonetheless, a few populations persisted in remote, high-country drainages to be rediscovered in the 1960s and 1970s.

In 1973, GCT from the few remaining populations were reintroduced into historic habitats. By 1998, the recovery plan noted that GCT occurred in 62 sites. Recovery efforts continue for this subspecies, which is listed as “threatened” pursuant to the Endangered Species Act.

Historic Range Relief Map

Our CSI analysis shows that GCT are still found in only a small portion of their native range, with 30 of 455 subwatersheds (6.6%) of the historic range currently occupied. Even within occupied subwatersheds, GCT often occur in restricted habitat areas such as a small lake or single stream segment. CSI scores for population extent were very low with only 3 subwatersheds (10%) scoring 3 or higher, indicating that most stream populations are 10-20 km of habitat or less. Most remaining populations are found at higher elevations, with a relatively high proportion of strongholds in Rocky Mountain National Park.

Total CSI scores vary widely, with only 10% of subwatersheds scoring in the highest category (81-90). Population density and genetic purity information were lacking for some populations (“starred” subwatersheds in Total CSI map).

Restoration actions should focus on 3 problem areas identified by the CSI: expanding and reconnecting small populations currently at risk of extinction, restoring migratory life histories, and controlling non-native trout in key subwatersheds. Interconnected genetically pure stream populations are needed as many existing populations are at risk of extinction because of their small size. Genetically, there are concerns from introgression of non-native cutthroat genes and rainbow trout genes as well as bottlenecks associated with small populations.

Key CSI Findings

• Less than 7% of historic range is occupied as measured by subwatersheds
• Most remaining populations are small and vulnerable to extirpation
• Primary strongholds exist in Rocky Mountain National Park
• Restoration and reintroduction efforts must address poor water quality and the presence of non-native trouts

In the management priorities map, most subwatersheds show high priority for reintroduction. Improvement in water quality, flows and control of non-native trouts are a prerequisite to reintroduction in many of these areas.

Prepared by Jack E. Williams, TU, 11/28/2006

Conservation Strategies Map

(500KB JPEG)