Year of Award


Document Type


Degree Type

Doctor of Philosophy (PhD)

Degree Name

Fish and Wildlife Biology

Department or School/College

W.A. Franke College of Forestry and Conservation

Committee Co-chair

Lisa Eby, Gordon Luikart

Commitee Members

Paul Lukacs, Ryan Kovach, Clint Muhlfeld


fitness, hybridization, introgression, RAD-seq, rainbow trout, westslope cutthroat trout


University of Montana


Human-mediated hybridization is a serious and growing threat to the conservation of biodiversity worldwide. Introgressive hybridization between introduced rainbow trout (Oncorhynchus mykiss, RBT) and westslope cutthroat trout (Oncorhynchus clarkii lewisi, WCT) is widespread across much of historical range of WCT and is considered one of the top threats to the persistence of remaining WCT populations. The ecological and evolutionary consequences of RBT hybridization and the role of environmental conditions in mediating those consequences is largely unknown but critical for making management decisions. I investigated three fitness-related traits across multiple populations to improve our understanding of the genetic basis of fitness differences due to RBT admixture. Specifically, I asked: how does RBT admixture affect growth, migratory life history, and survival; do environmental conditions mediate the effects of RBT admixture; and does RBT introgression at specific loci explain differences in these traits?

The environmental context in which hybridization occurs influences the outcomes of growth and survival, but not migratory behavior. I found that both growth and survival rates were variable among seasons and populations, likely due to the temporal and spatial environmental variation at these scales. While growth was significantly influenced by RBT admixture, this trait appeared to be less influential for determining the population-level consequences of hybridization as compared to survival and migratory behavior. Effects of RBT admixture on survival was highly variable, and ultimately reflected the population-level admixture. Interestingly, across these populations outbreeding depression was site-dependent and environmental gradients typically used for describing patterns in hybrid zones, such as thermal regimes, did not predict the variation in fitness consequences. A strong genomic association between RBT introgression on chromosome 29 and migratory behavior helps explain the rapid expansion of hybridization in systems through higher dispersal rates of hybrids. Overall, this research demonstrates that even low amounts of RBT admixture can substantially impact growth, survival and migratory behavior - all traits that can have large effects on population demography and evolution. This increases our knowledge of the impacts of RBT admixture on wild populations and provides valuable information regarding factors driving the spread of hybridization and local fitness consequences.



© Copyright 2020 Jeffrey Thomas Strait