YCT Multi-state Assessment February 10, 2003 



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

 arbitrary limit of admixture (introgressive hybridization) below which a population will be 

 considered 'pure' (e.g. 1%, 10%) are problematic because, as cited above, the amount of 

 admixture in many WCT populations is rapidly increasing. Research reporting the rapid spread 

 of introgression is significant and will have to be considered carefully by the agencies 

 responsible for managing these particular WCT populations. However, as reviewed previously, 

 it is highly unlikely that every WCT population that has experienced some level of hybridization 

 and introgression would experience an increase in the percentage of RBT introgression over time 

 or that introgression would spread rapidly from one population to many populations throughout a 

 drainage. Importantly, the reportedly continuing spread of RBT introgression within the 

 Flathead River system is likely due to the establishment of self-reproducing populations of 

 introduced rainbow trout and the dispersal of hybrids into areas containing pure cutthroat 

 populations (Hitt et al. submitted). In the case of the observed increase in hybridization and 

 introgression within the tributaries of the upper Kootenai River, those authors mention that "the 



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