YCT Multi-state Assessment February 10, 2003 



matings were incorrectly assumed to be operating for the above hypothetical mixed stock 

 population, genetic sample results would indicate introgression at levels in proportion to the 

 proportion of RBT to YCT for this hypothetical population. Where there was evidence of non- 

 random mating due to both the presence of some YCT individuals within the population that had 

 no evidence of introgression and evidence that reproductive isolation was occurring, YCT in 

 these stream segments were designated as a mixed stock that had both "genetically altered" and 

 "unaltered" individuals. This situation usually occurred in main rivers and mainstem tributaries 

 where it was known that YCT spawned in different tributaries or at different times than the 

 introgressing species (i.e. rainbow trout). However, when evidence to support non-random 

 mating was lacking, random mating was assumed. This assumption likely introduced 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 literamre. For our genetically unaltered ("pure") stream segments (code A; Table 1) 

 we selected less than 1% introgression, based on the most commonly defined level of 

 introgression that genetic sampling is designed to detect (Anon. 2000). The next two levels (i.e. 

 <=25% to >=1% and >25% introgression) were assigned based on conservation planning 

 considerations. The <=25% to >=1% classification was selected as the grouping that would 

 include conservation populafions that had specific unique attributes important for conservation. 

 Within this grouping are the stream segments that tested 90 to 99% YCT and there are 

 indications that meristic counts are not different between individuals ft"om populations that are 

 not genetically altered and those that are fi-om populafions with 10% or less introgression (Leary 

 et al. 1996). Stream segments that tested >25% might have appeared to be cutthroat trout to the 

 untrained eye, but they were treated as primarily having recreational value and were not earned 

 forward as conservation populations. The class where both hybrids and pure individuals inhabit 

 the same stream segment (code N; Table 1) indicated some reproductive isolafion and were 

 identified as a mixed stock 



Genetic information was extremely limited for some large geographic areas particularly in the 

 large tracts of wilderness, road-less, and National Park land in Montana, Idaho, and Wyoming. 

 The genetic characterization for the occupied stream segments in these areas were primarily 

 "suspected hybridized" or "potentially unaltered. We suspect that many of these segments 

 actually support genetically pure YCT. This question was addressed in the westslope cutthroat 

 status review that was completed in 2002 (Shepard et al. 2003) and we feel that a similar result 

 could exist for YCT. In addition, recent YCT population monitoring in southeastern Idaho 

 indicated that a high number of YCT judged to be genetically unaltered based on morphological 

 inspection were in fact genetically unaltered based on genetic test results (Meyer et al. 2003). 



Abundance Relative to Habitat Potential 



In addition to classifying genetic status, relafive abundance of YCT in qualitative terms (i.e. 

 abundant, common, rare or unknown) was also specified for each stream segment. For stream 

 segments where no quantified population determinations were available, habitat condition was 

 used as a surrogate to provide likel> abundance for each stream segment occupied by YCT 

 (Table 2; Appendix A). Relative abundance ranks based on habitat potential were rated in two 

 ways. First, where abundance data were available, that data was compared to all sites that had 

 similar habitat potential and rated V^y comparison. Data quality for these segments was usually 



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