WCT Multi-State Assessment February 10, 2003 



individuals within the population had no evidence of introgression. and biologists believed that 

 reproductive isolation occurred between stocks in a particular stream segment it was designated 

 as a mixed stock that had both "genetically altered" and "unaltered" individuals. However, when 

 there was no evidence to support non-random mating, random mating was assumed and this 

 likely introduced a bias toward classifying stream segments as introgressed when some may have 

 been mixed stock populations. 



The levels of introgression we assigned for genetically tested stream segments were based, in 

 part, on the literature. For our genetically unaltered ("pure") populations (code A; Table 1) we 

 selected < 1% introgression. based on the most commonly defined level of introgression that 

 genetic sampling is designed to detect. For the next level (90-99%; code B: 1 able 1) we relied 

 on the indication that meristic counts are not different between individuals from populations that 

 are not genetically altered and those that are from populations with 10% or less introgression 

 (Leary. Gould, and Sage 1996). The class where both hybrids and pure individuals inhabit the 

 same stream (code N; Table 1) indicated some reproductive isolation and more frequently 

 occurred in larger streams and rivers where spawning by WCT probably occurred in specific 

 headwater tributaries. The other two classes (codes C - 75-89% and D - <75%) were arbitrarily 

 assigned. 



Another major issue relates to whether introgression is natural (breeding between two native 

 taxa) or anthropogenic (introgression by normative species stocked by humans; Allendorf et al. 

 2001). Genetic testing does not normally distinguish whether introgression is natural or 

 anthropogenic; however, we reported all genetic results, regardless of the source of introgression. 

 For stream segments where no genetic testing occurred, we considered WCT as "suspected" 

 unaltered (code H; Table 1) if records indicated that no potentially introgressing species or 

 subspecies had been stocked or currently occurred, even if these WCT we're in sympatry with 

 native species that could potentially introgress with them. WCT in those stream segments where 

 potentially introgressing species or subspecies had been stocked or currently occurred were 

 classified as "potentially hybridized" (code J; Table 1) unless genetic testing found no evidence 

 of introgression. 



Since genetic information was extremely limited for some large geographic areas that had been 

 classified as both "potentially hybridized" and "suspected unaltered", particularly in the large 

 tracts of wilderness and roadless land in central Idaho, we compared the limited genetic testing ' 

 results that were available within a subset of these geographic areas to better evaluate potential 

 biases in these two classifications. We did this by comparing the proportion of genetic testing 

 results within each 4^^^ code FIUC that showed no introgression to the total area tested. The 

 proportion of stream miles containing unaltered and genetically tested WCT was then compared 

 to the proportions of miles of stream classified as "potentially hybridized" and "suspected pure" 

 to better display likely biases in these classifications within these HUCs. 



Abundance Relative to Habitat Potential 



In addition to recording the length of stream occupied by WCT, their relative abundance, as it 

 related to habitat "site potential", was rated as "at or near potential", "slightly below potential", 

 "significantly below potential", or "unknown" for each stream segment occupied by WCT (Table 

 2; Appendix B). These results were summarized by length of habitat occupied and not by 



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