WCT Multi-state Assessment February 1 0, 2003 



metapopulation. and many of the stream segments contained within this metapopulation were 

 genetically classified as "Potentially Altered". Genetic risk to this metapopulations was 

 considered to be relatively high, although genetic testing offish from portions of this 

 metapopulation suggests hybridization may not be widespread (Figure 5 and Table 10). 



Conclusions 



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

 distributed across, their historical range. WCT 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 Missouri River system of Montana. 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 fringe of a species range for many vertebrate species 

 (e.g. Brown 1984; Caughley et al. 1988; Lawton 1993). 



The precise genetic status of most WCT populations is uncertain because genetic testing is 

 expensive and time-consuming, thus genetic testing has not been completed for most occupied 

 stream segments. 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 information suggests that WCT with no evidence of 

 introgression currently occupy 13% of the habitats where WCT are currently found (8% of 

 historical). While it is probable that future 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 WCT 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 detemiine from 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. Over 1,000 miles of habitat supported WCT that biologists 

 knew were part of a mixed stock population (non-random mating) adding another 3% to the 

 proportion of currently occupied habitats that supported WCT where no evidence of 

 introgression was found (and another 2% to proportion of historical range). 



We contend that a minimum of 13% of the currently occupied habitats (8% of historical range) 

 should be considered as supporting genetically unaltered WCT. This contention is supported 

 both by the trends observed between assessments done over time, indicating that as more testing 

 is conducted, more streams are found that support unaltered WCT; and by the information 

 presented in this assessment indicating that in Idaho basins where limited genetic testing had 

 been done most testing found no evidence of introgression (Table 10). If we assume that half of 

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

 as "Potentially Altered" WCT are, in fact, supporting currently unaltered WCT, then the total 

 miles of likely unaltered WCT increases to over 12,500 miles (37% of currently occupied 

 habitats and 22% of historical range). In conclusion, we suspect that from 13 to 35% of habitats 

 currently occupied by WCT have not experienced genetic introgression (8 to 20% of historical 



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