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FWS Position Paper on Lineage GB populations outside the historic range of the <br />Greenback: Cutthroat Trout <br />Background <br />The greenback cutthroat trout (Onchorhj)nchzus clarki slon7icrs) was listed as an <br />endangered species in 1967, under a precursor to the Endangered Species Act (Act). It <br />was re-listed as endangered under the current Act in 19747 and downlisted to threatened <br />status, with a 4(d) rule allowing catch and release fishing, in 1978. <br />Greenbacks are considered native to the headwaters of the South Platte and Arkansas <br />River drainages in eastern Colorado, and a few headwater tributaries of the South Platte <br />in a small area of southeaster Wyoming (Behnke 1992). Another cutthroat trout <br />subspecies, the Colorado River cutthroat trout (Onchorhynchus clarki pleurilicus), occurs <br />in the Colorado and Green River drainages in the west slope of Colorado, southwestern <br />Wyoming, and eastern Utah. <br />Genetics and Taxonomic Issues <br />Since 2006, a number of genetic studies have been undertaken to try to determine the <br />genetic relationships between greenback, Colorado River, and Rio Grande cutthroat trout <br />(Onchorhynchus clarki 147- inalis) (Mitton et al. 2006, Metcalf et al. 2007, Metcalf 2007, <br />Rogers 2008). Mitton et al. (2006) found all 3 subspecies to be closely related, and did <br />not believe that any of them warranted subspecific designation. Metcalf et al. (2007) <br />used molecular markers from the mitochondrial and nuclear genomes to analyze <br />individuals from greenback and Colorado River cutthroat trout. Their studies revealed <br />two divergent lineages within the ranges of greenback and Colorado River cutthroat trout <br />consisting of 10 unique haplotypes, which they determined corresponded with the two <br />described subspecies. These lineages are known as GB (greenback) and CR (Colorado <br />River). Subsequent sampling and analysis found that of 45 assumed Colorado River <br />cutthroat populations, 12 were assigned to lineage GB. In addition, of 12 assumed <br />greenback populations present on the east side of the Continental Divide, 11 were <br />assigned to the CR lineage (Rogers 2008). Since publication of Rogers (2008), additional <br />sampling has identified additional lineage GB populations on the west slope of Colorado <br />and in eastern Utah (Rosenlund 2009 pers. corn). The current Rio Grande lineage <br />populations seem to fit well within the Rio Grande drainage. <br />To determine whether any taxonomic revisions to greenback: or Colorado River cutthroat <br />trout should be proposed, the Greenback Recovery Team is working on two research <br />projects: (l ) Attempting to see if the lineage markers can be found in the few cutthroat <br />trout samples collected in the 1800's, and currently stored in museums. and (2) <br />Attempting to determine if there is a physical difference between the three DNA <br />liricaues (greenback. Colorado River. and Rio Grande) through moipholog\ ? and meristics <br />analyses. Results of these projects are not expected for at least two years (Rosenlund <br />2009 pers. corn.).