YCT Multi-state Assessment February 10, 2003 



protecting relatively large areas of continuous habitat that allow YCT to express all life-history 

 traits, especially migratory life histories. As shown, the type of risks inherent in the two 

 different conservation strategies are dramatically different. 



For those YCT conservation populations where genetic integrity and isolation from competing 

 species is emphasized, risks due to isolation, small population size, and temporal variability are 

 high, while other types of risk are relatively low. The assumption made in rating these 

 population risks as high was that YCT populations benefit from occupancy in relatively large 

 habitats that allow for connection among subpopulations. Some authors have indicated that 

 cutthroat trout populations need to be supported by an effective population of 500 reproducing 

 adults based on the 50/500 "rule" (Franklin 1980; Soule 1980), thus they believed that most 

 isolated small populations of cutthroat trout were at an extremely high risk of extinction (Kruse 

 et al. 2001 ; Hilderbrand and Kershner 2000). Harig and Fausch (2001 ) found that cutthroat trout 

 translocations were most successftil when the drainage area was at least 5.6 mi.' (14.7 km'), 

 which likely translates to inhabited stream lengths of at least 2 to 3 miles. Hilderbrand and 

 Kershner (2000) estimated that cutthroat trout needed at least 5.7 miles (9.3 km) of habitat at 

 moderately high densities to persist under the 500 "rule". Rieman and Dunham (2000) provided 

 data that indicated small, isolated populations of WCT might not be as prone to extinction as 

 other vertebrates, and even other salmonids, based on their evaluation of the persistence of 

 isolated headwater populations of WCT in the Coeur d'Alene basin of Idaho. Of the 195 YCT 

 conservation populations we evaluated (Appendix F), 143 were considered as "isolates" with 

 majority having either moderately high or low composite risk scores ( weighted toward the 

 moderately high score). Risk factors of concern for most of the "isolates" were occupancy of 

 relatively small stream lengths (<10 km), smaller effective population sizes (fewer spawning 

 adults), and the potential detrimental influence of being isolated. None of the "isolate" 

 populations had a low composite risk score. Fifty-two (52) of the 195 designated conservation 

 populations were viewed as meta-populations consisting of several sub-populations having the 

 opportunity to interact. Most meta-population composite risk scores were at the low end of the 

 moderate score. The risk factor of most concern for the meta-populations was loss of genetic 

 identity. There were 1 1 meta-populations that had low composite risk scores. There were a 

 significantly higher number of "core" conservation populations that were identified as "isolates" 

 (95%). It is anticipated that the number of "core" conservafion populafions will increase 

 substantially as more genetic testing is accomplished. There could be as many as 74 additional 

 core conservation populations dependant upon the results of the genetic testing yet to be 

 preformed. 



Since genetic introgression and nonnative competition threats probably outweigh stochastic risks 

 over the short-term for many extant YCT populations, isolating remaining non-introgressed YCT 

 populations may be a prudent, short-term conservation strategy. Replicating and re-founding 

 existing isolated, non-introgressed YCT populations that may be lost due to stochastic or 

 demographic pressures, and using humans as the dispersal agent via conservation stocking to re- 

 found YCT populations that are lost from isolated habitats due to stochastic processes have been 

 recognized as viable conservation strategies (e.g. Montana Fish, Wildlife and Parks 1999; 

 Shepard et al. in press). 



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