hybrid swarm. First, continizency table chi-square analysis was used to test for heterogeneity of allele frequencies 

 among the marker loci. Ne.xt, we compared the observed distribution of the number of loci per individual at 

 which non-native fragments were detected to the expected random binomial distribution based on the estimated 

 native and non-native genetic contributions to the population. If both analyses were non-significant ve 

 concluded the population came from a hybrid swarm. , . ,,. .... ■ . .; 



Heterogeneity of allele frequencies among marker loci can arise in very old hybrid swarms as the frequencies 

 over time diverge from each other due to genetic drift. In this case, however, the non-native fragments will still 

 be randomly distributed among individuals. 



There are two likely reasons why a non-random distribution of non-native fragments may be observed among 

 individuals in a sample. It may contain individuals from genetically divergent populations with different amounts 

 of hybridization or hybridization may have only recently occurred in the population. Based on genetic data alone, 

 these two situations will generally be difficult to distinguish from each other. Regardless of the explanation, 

 when the non-native fragments are not randomly distributed among individuals in a sample estimating a mean 

 level of hybridization has little, if any, biological meaning and, therefore, is often not estimated. 



Results and Discussion: 



Deep Creek 3087 



PINE fragments characteristic of only westslope cutthroat trout were detected in the sample. A previous allozyme 

 analysis of 26 fish (#422) also detected alleles characteristic of only westslope cutthroat trout. When these two 

 samples are combined, we have better than a 99% chance of detecting as little as a one percent rainbow or Yellowstone 

 cutthroat trout genetic contribution to a hybrid swarm. This population, therefore, is very likely non-hybridized 

 westslope cutthroat trout. 



Park Creek 3088 



PINE fragments characteristic of only westslope cutthroat trout were detected in the sample. With a sample size of 

 22 individuals, we have only a 93% chance of detecting as little as a one percent rainbow trout genetic contribution 

 and an 83% chance of detecting as little as a one percent Yellowstone cutthroat trout genetic contribution to a hybrid 

 swarm. Thus, we cannot reasonably exclude the possibility that this population may be slightly hybridized with 

 rainbow trout, Yellowstone cutthroat trout, or both. Unless future data indicate otherwise, however, the conservative 

 approach would be to consider this a non-hybridized westslope cutthroat trout population. 



Blackfoot River 3089 



Genetically, this sample contained three noteworthy fish. Individual 1734 possessed PINE fragments characteristic of 

 rainbow trout at all si.x diagnostic loci for this fish that were analyzed, and possessed no PINE fragments characteristic 

 of westslope cutthroat trout at the seven diagnostic loci for this fish that were analyzed. This suggests this individual 

 may be a non-hybridized rainbow trout. Individual 1 840 possessed all six fragments characteristic of rainbow trout 

 and all seven fragments characteristic of westslope cutthroat trout suggesting it is very likely a first generation hybrid 

 between these fishes. Individual 1950 possessed all seven fragments characteristic of westslope cutthroat trout, but 

 possessed fragments characteristic of rainbow trout at only four of the six diagnostic loci for this species that were 

 analyzed. Thus, individual 1 950 appears to be a first generation backcross to westslope cutthroat trout; that is, the 

 progeny of a mating between a first generation hybrid and a westslope cutthroat trout. All the other fish in the sample 

 (1731-1733, 1735-1737, 1838, 1839, 1841-1848, and 1949), possessed fragments characteristic of rainbow trout at 

 five or six diagnostic loci and fragments characteristic of westslope cutthroat trout at one or two diagnostic loci. These 

 individuals, therefore, appear to be late generation hybrids between rainbow and westslope cutthroat trout with a 

 predominant rainbow trout genetic contribution. Thus, overall this sample appears to have contained a mixture of 



