YCT Multi-state Assessment February 1 0, 2003 



there is no evidence the sample consists of more than one population. Certainly in the cases of 

 sympatric populations of native RBT and native WCT, even those in which a certain level of 

 hybridization and introgression has occurred, the documentation of the percentage RBT alleles 

 out of the total examined does not accurately describe the status of the population. The same is 

 true in situations where Fl hybrids are observed, but no backcross hybrids are observed. For 

 instance, if 30 individuals are sampled, and 10 of them have genotypes indicative of Fl hybrids, 

 10 have genotypes indicative of WCT, and 10 have genotypes indicative of RBT, the results 

 could be interpreted to say the population is introgressed at a level of 50%, when in fact, these 

 results demonstrate no RBT introgression. This particular situation would be important to 

 document and manage since it represents a loss in reproductive effort for both species, but it has 

 very different management and conservation implications than a hybrid swarm consisting of a 

 mixture of 50% WCT alleles and 50% RBT alleles. A more informative way of describing 

 hybridization and introgression within sympatric populations is to first delineate populations and 

 then to describe the observed genotypes and their ft^equencies within those populations. 



References 



Hitt, N.P, C. A. Frissell, C. Muhlfeld, and F. W. Allendorf. Submitted. Spread of hybridization 

 between native westslope cutthroat trout, Oncorhynchus clarki lewisi, and non-native 

 rainbow trout, O. mykiss, in western Montana. Canadian Journal of Fisheries and Aquatic 

 Sciences. 



Leary, R. F., F. W. Allendorf, and G. K. Sage. 1995. Hybridization and introgression between 

 introduced and native fish. Amer. Fish. Soc. Symposium 15:91-101. 



Mays, J.D. 2002. RAPD and mitochondrial DNA analyses of cutthroat trout ft^om four streams 

 in the Salmon River Breaks, Idaho. Thesis. University of Idaho. Department of Fish and 

 Wildlife Resources. Moscow, ID. 



Rubidge E., P. Corbett, E.B. Taylor.2001. A molecular analysis of hybridization between native 

 westslope cutthroat trout and introduced rainbow trout in southeastern British Columbia, 

 Canada. J. Fish. Biol. 59:42-45. 



Sage, G.K., R.F. Leary, and F.W. Allendorf. 1992. Genefic analysis of 45 populations in the 

 Yaak River Drainage, Montana. Wild Trout and Salmon Genetics Laboratory Report 

 92/3, University of Montana, Missoula. 



Weigel, D.E., J.T. Peterson, P. Spruell. 2002. A model using phenotypic characterisfics to detect 

 introgressive hybridization in wild westslope cutthroat trout and rainbow trout. Trans. 

 Am. Fish. Soc. 131:389-403. 



Williams, R. N., D. K. Shiozawa, J. E. Carter, and R. F. Leary. 1996. Genetic detection of 



putative hybridization between native and introduced rainbow trout populations of the 

 Upper Snake River. Transactions of the American Fisheries Society 125:387-401. 



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