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 Chair

Andrew Whiteley

Commitee Members

Lisa Eby, Paul Lukacs, Gordon Luikart, Ryan Kovach


Climate change, Cutthroat trout, Effective population size, Genetic rescue, Inbreeding depression, Invasive species


University of Montana


Anthropogenic habitat destruction has isolated innumerable populations that now face increased extinction risk due to demographic and genetic factors. Although often the best strategy, restoring connectivity can be challenging or even harmful. Such is the case for westslope cutthroat trout (Oncorhynchus clarkii lewisi; WCT) in the Missouri River basin, which are limited to completely isolated populations. Nonnative species threaten WCT in connected watersheds and barrier removal could be detrimental. My dissertation examines trade-offs and strategies for the management of isolated WCT. I first examined how nonnative trout species and climate change influence the distribution of WCT using a multispecies, dynamic occupancy model parameterized with 21,917 surveys collected over 30 years. I predicted that the future distribution of WCT will decline by 16%, primarily due to warming water increasing the distributions of harmful nonnative species. I next asked whether genetic metrics indicated that isolated WCT populations are at risk of inbreeding depression. I found very low effective population sizes (Ne < 25) in two of five WCT populations, suggesting risks of inbreeding depression could be high. A promising conservation strategy is to restore gene flow into small populations, which can increase vital rates and, ultimately, persistence probability (i.e., genetic rescue). To examine genetic rescue as a conservation strategy for WCT, we first conducted a literature review to examine what aspects of genetic rescue remain uncertain, including the duration and magnitude of genetic rescue and when gene flow may reduce fitness. Finally, we conducted an experimental test of genetic rescue in four isolated WCT populations. In the two smallest populations, we found that F1 hybrids had a 71% and 379% increase in fitness relative to residents, suggesting genetic rescue occurred. However, in the two larger populations, we found minimal evidence for genetic rescue. Overall, this research demonstrates that isolation likely poses risk to WCT, but removing barriers could pose a far greater risk owing to increased interactions with nonnative trout species. These results provide further evidence that when restoring connectivity is not an option genetic rescue is a powerful conservation strategy for at-risk populations of diverse taxa.



© Copyright 2022 Donovan Alexander Bell