YCT Multi-state Assessment February 10, 2003 



Executive Summary 



The distribution and abundance of Yellowstone cutthroat trout {Oncorhynchus clarki houvieri: 

 YCT) have declined from historical levels over part or all of their 1 range. For this YCT 

 assessment we used existing infomiation provided by 43 fisheries professionals applied through 

 a consistent methodology to assess the extent of YCT historical range, their current distribution, 

 including genetic status, and evaluated the foreseeable risks to 1 95 populations designated as 

 "conservation populations" by management agencies. 



We estimated that YCT historically occupied about 17,397 miles of habitat in the western U.S. 

 YCT currently occupy an estimated 7,528 miles of historical habitats (43%). Genetic testing has 

 been completed across about 1,880 miles of habitat (25% of occupied habitats), but sample sizes 

 were variable and sample sizes of 25 fish or more (a sample size that likely would detect as little 

 as 1% introgression at alpha = 0.05) made up 27% of the samples. YCT with no evidence of 

 genetic introgression currently occupy about 1,300 stream miles ( \1%) of habitat. Another 105 

 miles of currently occupied habitats (1%) contained YCT that were identified as part of a mixed 

 stock where the YCT were not introgressed. We suggest that even though genetic sampling was 

 nonrandom because sampling likely occurred more frequently in YCT populations that appeared 

 non-introgressed, some, if not much, of the habitats currently occupied by YCT with no genetic 

 testing likely support populations that are not introgressed. Approximately 3,000 miles of 

 occupied habitat were identified as containing genetically unaltered YCT based on no record of 

 stocking or by having no hybridizing species present. Much of the habitat currently occupied by 

 YCT was located in designated parks (2%), wilderness areas (19%), and other road-less areas 

 (40''/o), and almost 70'''o of habitats currently occupied lie within federally managed lands. 



A total of 195 separate YCT populations currently occupying 6,346 miles of habitat were 

 designated as "conservation populations" (84.3% of currently occupied habitat). These 

 conservation populations were spread throughout the historical range, occuring in 35 of the 41 

 liydrologic units historically occupied by YCT. Most of these conservation populations were 

 judged to be "isolets" (143 or 73%); however, meta-populations occupied much more of the 

 habitat (5,515 miles or 73%). Of the 195 designated conservation populations, 133 (68%) had at 

 least some component that was tested as genetically unaltered or viewed as being potentially 

 unaltered. Thirty-nine populations (20%) consisted of only tested unaltered YCT. More isolet 

 populations were at higher risks due to temporal variability, population size, and isolation than 

 mcta-populations, but these isolets were generally at less risk from genetic and disease factors 

 than meta-populations. 



This assessment clearly shows that YCT currently occupy significant portions of, and are well 

 distributed across, their historical range. The data suggest that genetically unaltered YCT 

 occupy at least 1 7"o and possibly up to 58'!o of currently occupied habitats (7 to 25°o of 

 historical habitats). Conservation population designations suggest that two ditTerent 

 conservation management strategies are needed and being implemented to conserve YCT. One 

 strategy concentrates on preventing introgression, disease and competition risks through isolation 

 of YCT, while the other concentrates on preserving meta-population function and multiple life- 

 history strategies by connecting occupied habitats. 



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