WCT Multi-state Assessment February 1 0. 2003 



and hybrids, and the amount of natural gene flow that occurs between WCT populations. If 

 stocking tootc place in areas that had not been stocked prior to the first study, then subsequent re- 

 sampling and genetic analysis may find an increase in the number of populations that show 

 introgressive hybridization. If RBT are introduced into an area with WCT and there is 

 subsequent introgressive hybridization, gene flow will move RBT alleles into surrounding 

 populations. In some areas, stocking has resulted in self-sustaining RBT populations (Hitt et al. 

 submitted). If these introduced populations increase in size and/or individuals disperse and 

 irrmiigrate. both the percentage of RBT alleles within populations, as well as the number of 

 mtrogressed populations can increase, if those immigrants are reproductively successful. 



It is important that managers continue to screen WCT populations for hybridization and 

 introgression and they also continue to investigate the ecological and genetic factors that 

 influence the consequences of non-native introductions. In some cases the outcome of stocking 

 non-native trout on indigenous WCT populations has been severe enough as to have led to the 

 formation of hybrid swarms (Hitt et al. submitted). However, it is likely that a number of factors, 

 including existing reproductive isolating mechanisms (e.g. those found in naturally sympatric 

 populations) or environmental conditions which select against non-native trout and hybrids, have 

 limited the incidence of hybridization and spread of introgression in a number of drainages, and 

 has thus preserved genetic integrity of the native parental populations. This is not to suggest that 

 the practice of stocking fertile, non-native trout on indigenous WCT populations should 

 continue. The States of Idaho, Montana. Oregon, and Washington have already adopted policies 

 focused either on the cessation of stocking non-native trout in WCT waters, or the use of sterile 

 triploid rainbow trout in hatchery supported fisheries which are adjacent or connected to waters 

 supporting westslope cutthroat trout. 



It is also important that managers monitor and document possible changes in the level of 

 introgression within a population or changes in the number of populations in which hybridization 

 and introgression is observed. Populations in which introgression has increased over time should 

 not receive the same conservation status and should be managed differently than populations in 

 which introgression levels have remained stable or are decreasing. Documenting areas in which 

 population-level introgression is increasing or where the number of hybridized populations is 

 increasing is essential because it may highlight areas in which management actions should 

 change (e.g. stopping further introductions of hatchery rainbow trout. Rubidge et al. 2001). 



Ideally, research studies that examine temporal changes among vagile animals should attempt to 

 compare samples collected from the exact same location and at the same time of year. 

 Additionally, samples sizes should be similar and the genetic methods used should be similar in 

 their precision and accuracy of detecting hybridization and introgression. Preferably, the exact 

 same diagnostic loci would be used so that frequencies of specific diagnostic alleles could be 

 monitored over time in the population. 



Recent research in the Flathead River system in Montana (Hitt et al. submitted), and in the 

 Kootenay River drainage in British Columbia (Rubidge et al. 2001 ) has reported the rapid 

 spread of RBT introgression into WCT populations previously reported as free from detectable 

 levels of introgressive hybridization. Some researchers, who have addressed the question of how 

 to define a "pure' WCT population, have argued that management plans that attempt to set some 



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