Greenback Cutthroat Trout – Genetic Study
Introduction to the Genetic study (Metcalf 12028)
by Allyn Kratz
This study compared DNA samples of Trout Specimens collected and preserved between 1857 and 1889 to those populations of trout residing in Colorado today. These samples were collected from museums throughout the United States. (As a side note, Dr Metcalf stated during a presentation the the Greenback Recovery Team, that it was easier to collect DNA from the bones of animals dead for more than 50,000 years then it was from the samples of trout preserved in Ethanol only 150 year ago.)
Historically, it has been thought that there were four subspecies of Cutthroat Trout native to Colorado. The Colorado River Cutthroat residing in the Yampa, Colorado, Gunnison and San Juan River basins, the Rio Grande Cutthroat residing in the Rio Grande river basin, the Yellow Fin Cutthroat Trout residing in the Arkansas River, primarily around the Twin Lakes region and the Colorado Greenback Cutthroat Trout residing in the Arkansas and South Platte River drainages.
The Yellow Fin Cutthroat Trout has been declared extinct since 1904 while the Colorado Greenback Cutthroat Trout was thought to be extinct in the mid 1950’s. Since that time populations of trout thought to be Colorado Greenback Trout have been discovered with attempts to propagate and recover them from extinction. As science developed these populations of trout were discovered to be hybridize with Rainbow Trout or as this study points out, the most recent recovery work has actually been propagating Colorado River Cutthroat Trout.
This confusion was created when man began to move trout around the state. State and Federal government records show that as early as 1873 trout were being raised and planted around the state. The initial reason for this hatchery efforts was due to the decline in the population of the Native Cutthroat Trout due to over fishing, Mining and Agriculture activities These trout were primarily Rainbow, Brook and Native Cutthroat Trout. The Cutthroat primarily raised in the hatcheries were collected from the Western Slope. Very few Trout were collected from the Eastern slope for propagation and subsequent planting. This probably added to the disappearance of the Greenback Trout along with the fact the Brook Trout can better compete and Rainbow Trout hybridize with the Greenback Trout. During the period of time from 1899 to 1909, 29,000,000 trout were raised in federal facilities and stocked into lakes and streams around Colorado. From 1914 – 1925, 26,000,000 trout were raised and stocked.
When the museum samples were compared there appeared to be six DNA gene cluster types. Each of these clusters were collected from a different river basins. For the purpose of identification, each cluster type was designated with a color. The Blue samples were all collected from the Yampa basin, the Purple were all collected from the South Platte basin, the Yellow were all from the Arkansas River basin, the Orange were all from the Rio Grande River basin, and the Red were all collected from the San Juan. The green were different as they were collected from the Colorado, the Gunnison and the Twin Lakes area of the Arkansas. This might be explained by the fact that these museum samples were originally collected in 1889, after the movement of hatchery fish in Colorado had begun. The Leadville Hatchery is close to the Twin Lakes area of the Arkansas.
These DNA gene clusters were then compared to the modern day Cutthroat Trout. The
Orange cluster compared favorable with the modern day residents of the Rio Grande drainage with the conclusion that these are O. c. virginalis the Rio Grande Cutthroat Trout. The museum samples of the Yellow clusters were found throughout the Arkansas River basin and not just the Twin Lakes area. These would have been the Oncorhynchus clarkii macdonaldi and no modern day specimens have been found. These were apparently the extinct Yellow Fin Cutthroat Trout. In addition, no modern day specimens could be found that are consistent with the Red DNA gene cluster and found in the San Juan basin, the conclusion being that this subspecies is also extinct.
The Blue gene cluster is consistent with the modern day cutthroat that appears to have been extensively propagated and stocked in lakes and streams around Colorado. The greatest population of these modern day fish are found in the Yampa Valley and the author argues that these fish should them receive the name O. c. pleuriticus. The Green cluster also found extensively around the state but primarily in the Gunnison and Colorado basins is thus with out a name and one of the new subspecies identified by this study. The population of Cutthroat Trout found in Severy Creek belongs to this group.
That leaves the Purple DNA gene cluster, originally found only in the South Platte drainage O. c. stomias, and now found only in the Bear Creek near Colorado Springs, which oddly is in the Arkansas drainage. View a video of these Greenback Trout.
Currently, O. c. stomias appears to persist as a single self-sustaining population in a
locality outside the native range of the subspecies. The population harbors little genetic variation for loci that are typically variable in cutthroat trout populations (Metcalf et al. 2007) probably the result of few founding fish used in the initial stocking effort or a subsequent population bottleneck.(Metcalf et al. 2012)
As a result of this genetics study, a morphology study was done on the Cutthroat Trout found in Colorado. These two studies have been sent to the US Fish and Wildlife Service for review and determination of not only to designate their protection level under the endangered species act but also to resolve the issue of whether there were Six subspecies of Cutthroat Trout in Colorado or some other number.