PINE fragments characteristic of rainbow trout were detected at four of the six diagnostic 

 loci analyzed that usually distinguish rainbow trout from westslope cutthroat trout. As in 

 the previous two samples, however, the fragments characteristic of rainbow trout were 

 not randomly distributed (P<0.001) among the fish in the Fish Creek sample. Rather, 

 they were detected in only three fish. Of these, two possessed a rainbow trout fragment 

 at only one locus, but the particular locus differed between the two. The other fish 

 definitely of hybrid origin possessed rainbow trout fragments at three diagnostic loci. 

 The remaining 22 fish in the sample possessed PINE fragments characteristic of 

 westslope cutthroat trout at all loci analyzed. The simplest explanation compatible with 

 these data is the sample contained individuals from two genetically different populations. 

 The majority of the fish appear to have come from a westslope cutthroatXrainbow trout 

 hybrid swarm with a predominant westslope cutthroat trout genetic contribution. A small 

 proportion of the fish appear to have come from a hybridized population of westslope 

 cutthroat and rainbow trout with a much more substantial rainbow trout genetic 

 contribution than the former population. These latter fish may well be recent migrants or 

 theprogeny of recent migrants into the population, m - .- ■ i . i 



Skalkaho Creek (#537, #899, #2312, and #2923) 



The first two samples collected from Skalkaho Creek (537, collected September 3, 1991, 

 NIO; and 899, May 3, 1994, 10; both from T5N R19W S27) came from the very upper 

 reaches. AUozyme analysis indicated no evidence of hybridization and, therefore, the 

 samples were reported as appearing to have come from non-hybridized westslope 

 cutthroat trout. There was no evidence of genetic differences between the samples 

 (contingency table Chi-square, P>0.05) so they were combined into one. With the 

 combined sample size of 20, there was about a 91 % chance of detecting as little as a one 

 percent rainbow trout and a 98% chance of detecting as little as a one percent 

 Yellowstone cutthroat trout genetic contribution to a hybrid swarm. At the time these 

 samples were collected, therefore, upper Skalkaho Creek very likely contained a non- 

 hybridized westslope cutthroat trout population. 



The fish in sample 2312 (September 10, 2002, 25) were collected from Ward Ditch. 

 PINE fragments characteristic of rainbow trout were detected at four of the six diagnostic 

 loci analyzed that usually distinguish rainbow trout from westslope cutthroat trout. In the 

 original report, the sample was considered to have come from a westslope 

 cutthroatXrainbow trout hybrid swarm. This, however, does not appear to be the case as 

 the rainbow trout PINE fragments were not randomly distributed (P<0.001) among the 

 fish in the sample. In contrast, they were detected in only two individuals with one 

 possessing rainbow trout markers at three diagnostic loci and the other at one. The 

 remaining fish in the sample possessed PINE fragments characteristic of only westslope 

 cutthroat trout. These results are compatible with at least a couple of explanations. First, 

 the fish sampled may have been a mixture of non-hybridized westslope cutthroat frout 

 and some fish of hybrid origin. Conversely, the fish may have been a mixture of 

 individuals from a westslope cutthroatXrainbow trout hybrid swarm with a small rainbow 

 trout genetic contribution and migrants from a hybridized population between westslope 



