WCT Multi-state Assessment February 10, 2003 



working with cutthroat trout. Cutthroat trout with a measured introgression level of less than 1 % 

 are designated as "core conservation populations", and are considered pure. The less than 1% 

 limit allows for possible imprecision associated with genetic markers. A second category, 

 "conservation population", is used for populations with less than 10% introgression (but may 

 extend to a greater amount depending upon circumstances and the values and attributes to be 

 preserved). The less than 1 0% criterion is not suggesting that populations with introgression 

 levels between 1% and 10% be considered 'pure" or managed as a 'pure* populations, rather it is 

 an agreed upon decision to manage populations a certain way given that a particular level of 

 iuuogiession is observed (in this case, <10%). Importantly, the primary management goal of the 

 "conservation population" designation is to protect and conserve populations that, while existing 

 in a introgressed condition, still contain a unique or essential portion of ecological, behavioral, 

 physiological, or genetic diversity found within the subspecies. 



A concern with setting such threshold criteria based on percentages is that those criteria may not 

 accurately describe the true hybridization status of a sample location. The percentage 

 corresponds to the number of non-native allele observed among the total alleles examined, and 

 is only useful in situations where the researcher is using dominant markers and can detennine 

 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 frequencies 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 from four streams 

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

 Wildlife Resources. Moscow, ID. 



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