Year of Award


Document Type


Degree Type

Master of Science (MS)

Degree Name

Wildlife Biology

Department or School/College

W.A. Franke College of Forestry and Conservation

Committee Chair

Lisa Eby

Commitee Members

Ryan Kovach, Mike Mitchell


trout, hybridization, evolutionary forces, spawning, partial migration, rock creek


University of Montana

Subject Categories

Terrestrial and Aquatic Ecology


Westslope cutthroat trout Oncorhynchus clarkii lewisi (WCT) is a native species of high conservation value that historically exhibited partially migratory behavior throughout its range. Long-term persistence of WCT is threatened by human habitat modification, fragmentation, introduction of non-native species, and hybridization. As a result of these changes, remnant populations in Montana have shifted toward resident populations in headwater systems and away from migratory populations in larger connected river networks. This is compounded by the historic introduction of rainbow trout O. mykiss (RBT) that hybridize with WCT, especially in larger river habitats. Rock Creek in western Montana, USA was historically managed as a world class RBT fishery. Despite intensive pressure from non-native species, non-hybridized migratory WCT still exist within Rock Creek. Understanding the drivers and mechanisms that have maintained this non-hybridized population of partially migratory WCT is of high importance to managers trying to manage populations in the face of multiple threats to their persistence.

First, we investigated the migratory life history of WCT in Rock Creek, including the diversity of behaviors within the population, potential drivers promoting it, and threats to this population. Second, we investigated dynamics of hybridization between RBT and WCT; specifically, what factors are shaping the pattern of hybridization across the landscape and what mechanisms are providing resistance to hybridization? Our results revealed more than a 20-fold variation in spawning migrations distances among individuals, with migratory fish returning to thirteen different tributaries. Migratory behavior was associated with larger spawning tributaries with higher adult biomass. Longer migrating individuals spawned lower in tributaries where there was greater overlap with RBTxWCT hybrids. Survival was low and not related to spawning or migration distance but was strongly related to summer habitat. Propagule pressure was the main mechanism driving the pattern of hybridization. Sites that were resistant to hybridization were further from the highest source of RBT, had more spatial and temporal assortative mating, and larger resident individuals. When propagule pressure is high it likely overwhelms localized resistance. Management and conservation of WCT, and similar partially migratory species, in these connected stream networks requires a basin scale approach that prioritizes connectivity among habitats, promotes natural flow regimes, and works to reduce abundances of non-native species.



© Copyright 2021 Troy Weant Smith