YCT Multi-state Assessment Februar>' 1 0, 2003 



Land-use implications and application of actions focused on improving the conservation 

 potential of the conservation populations is viewed was important but due to limited information 

 and lack of consistency in the information currently available, the significance of these factors 

 was not validated effectively in our assessment. We did make an effort to acknowledge which 

 land uses were felt to be influencing the populations and to identify the various conservation 

 actions that had been applied to the conservation populations. 



Conclusions 



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

 distributed across, their historical range. YCT currently occupy a higher proportion of their 

 historical habitats near the core of their historical range, with sparser occupancy near range 

 fringe areas, particularly in the Snake River system of Idaho below the Portneuf River and in the 

 middle Yellowstone River and lower Bighorn River systems. Several studies, both theoretical 

 and empirical, have suggested a decline in the proportion of sites occupied and in population 

 densities from the center to the fiinge of a species range for many vertebrate species (e.g. Brown 

 1984; Caughley et al. 1988; Lawton 1993). Meyer et al. (2003) observed that most YCT 

 populations in southeastern Idaho had neither declined in abundance nor distribution over the last 

 two decades. 



The precise genetic status of most YCT populations is uncertain because genetic testing has not 

 been completed for a substantial number of populations. Genetic testing is expensive and time- 

 consuming. Efforts to gain more genetic information will continue consistent with fiinding and 

 available workforce. Also, even for some populations where genetic testing has been completed, 

 sample sizes are so small that the absence of introgression cannot be statistically inferred with 

 any degree of confidence. Existing genetic infonnation suggests that YCT with no evidence of 

 introgression currently occupy 17% of the habitats where YCT are currently found (7% of 

 historical). While it is probable that fijture evidence of introgression will be found in some of the 

 populations that currently have shown no evidence of introgression, it is also likely that more of 

 the currently untested populations of YCT will be found to have no evidence of introgression, 

 once they are genetically tested. 



In addition, we know that the data were biased because stream segments were assigned as 

 introgressed when we could not determine fi-om the database whether a particular sample was 

 from a population where random or non-random mating occurred. Thus, unless a biologist or 

 geneticist knew that non-random mating occurred, we assumed random mating had occurred for 

 all genetic samples where introgression was detected and the level of introgression was 

 computed based on that assumption. 



We are reasonably sure that a minimum of 17% of the currently occupied habitats (7% of 

 historical range) should be considered as supporting genetically unaltered YCT. This contention 

 is supported both by the trends observed between assessments done over time, indicating that as 

 more testing was conducted, more streams were found that support unaltered YCT; and by the 

 information cited in this assessment indicating that in Idalio basins where limited genetic testing 

 had been done, most testing found no evidence of introgression. If we assume that half of the 

 area that we classified as supporting "Potentially Unaltered" and 20% of the areas classified as 

 "Suspected Altered" YCT are, in fact, supporting unaltered YCT, then the total miles of likely 



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