WCT Multi-state Assessment February 10, 2003 



Executive Summary 



The distribution and abundance of westslope cutthroat trout {Oncorhynchus clarki lewisi; WCT) 

 have decHned from historical levels over part or all of their historical range. For the U.S. range 

 of WCT we used existing information provided by 1 12 fisheries professionals applied through a 

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

 hiciuding genetic status, and evaluated the foreseeable risks to 539 populations designated as 

 "conservation populations" by management agencies. 



We estimated that WCT historically occupied about 56,500 miles of habitat within the U.S. 

 WCT currently occupy an estimated 33,500 miles of historically occupied habitats (59%). 

 Genetic testing has been completed across about 6,100 miles of habitat (18% 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% levels of introgression with a 95% level of confidence) made 

 up 30% of the samples. WCT with no evidence of genetic introgression currently occupied 

 about 3,400 miles (10%) of currently occupied habitats. Another 1,000 miles of cuiTently 

 occupied habitats (3%) contained WCT that were probably part of a mixed stock where the WCT 

 were not introgressed. We suggest that even though genetic sampling was nonrandom because 

 sampling likely occurred more frequently in WCT populations that appeared non-introgressed, 

 some, if not much, of the habitats currently occupied by WCT where no genetic testing has been 

 done likely support populations that are not introgressed. Much of the habitat currently occupied 

 by WCT was located in designated parks (2%), wilderness areas (19%), and roadless areas 

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



A total of 563 separate WCT populations currently occupying 24,450 miles of habitat were 

 designated as "conservation populations". These conservation populations were spread 

 throughout the historical range, occuring in 67 of the 70 hydrologic units historically occupied 

 by WCT. Most of these conservation populations were believed to be "isolets" (457 or 81%); 

 however, metapopulations occupied much more of the habitat (21,600 miles or 88%). Of the 563 

 designated conservation populations, 339 (60%) had at least some component that was 

 genetically unaltered and 172 (30%) consisted entirely of stream segments that were genetically 

 uiialtered. For the 539 conservation populations for which risks to the population were 

 asseessed, more isolet populations were at higher risks due to temporal variability, population 

 size, and isolation than metapopulations, but these isolets were generally at less risk from genetic 

 and disease factors than metapopulations. 



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

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

 occupy at least 13% and possibly up to 35% of currently occupied habitats (8 to 20% of 

 historical habitats). Conservation population designations suggest that two different 

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

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

 of WCT, while the other concentrates on preserving metapopulation function and multiple life- 

 history strategies by connecting occupied habitats. 



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