WCT Multi-State Assessment February 10, 2003 



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 

 (2002) found that cutthroat trout translocations were most successful 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 

 Dunliam (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 172 conservation populations we evaluated that consisted exclusively of WCT with no 

 evidence of introgression, 39 (23%) occupied more than 5.5 miles, 105 (61%) occupied more 

 than 3 miles, and 132 (77%) occupied 2 miles or more. Conservation populations that were 

 considered "'isolets" accounted for 168 (98%) of these 172 conservation populations with no 

 evidence of introgression. Conservation of genetic integrity was the reason for designating 161 

 (94%) of these populations that had no evidence of introgression as conservation populations and 

 most (157 or 98%) were "isolets". Of the 168 "isolef conservation populations that had no 

 evidence of introgression, 56 (33%) were genetically secured by the presence of a barrier. 



Since genetic introgression and nonnative competition threats probably outweigh stochastic risks 

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

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

 founding existing isolated, non-introgressed WCT populations that may be lost due to stochastic 

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

 re-found WCT 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). 



For the other type of "metapopulation" conservation population, population risks are relatively 

 low, while genetic and disease risks are high. While 70 of the 106 conservation populations 

 classed as "metapopulations" had a component that had no evidence of genetic introgression. 

 only four consisted exclusively of nonintrogressed WCT. These four occupied relatively short 

 lengths of habitat (2.8 to 8.3 miles), consequently they may have been misclassified as 

 "metapopulations". 



Long-term abundance data from four Idaho systems located in central Idaho (Middle Fork 

 Salmon, Selway, St. Joe and Coeur d'Alene rivers) indicated a high level of resiliency for 

 populations in these basins (Idaho Department of Fish Game 2003), providing further evidence 

 that population-level risks are likely low for conservation populations designated as 

 metapopulations. The Middle Fork Salmon and Selway rivers are primarily in wilderness areas, 

 and drain the unproductive granitic geology of the Idaho batholith, while the St. Joe and Coeur 

 d'Alene systems drain more productive belt series geologies. The IDFG (2003) data also 

 supported the "at or near capacity" abundance designation assigned to most wilderness stream 

 segments and the "significantly below capacity" designation for many of the stream segments in 

 the Coeur d'Alene system. WCT from these two rivers were included within a single large 



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