Date of Award

Spring 2023


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Document Type


Degree Name

Master of Science (MS) in Biology




Iteroparous salmonids that exhibit a migratory life history are essential to functioning metapopulations. They are demographically important as migratory females produce more eggs than non-migratory individuals. Additionally, they provide genetic support through gene flow, resulting in more robust, genetically diverse populations. The lower Priest River flows into the Pend Oreille River in the panhandle of northern Idaho and is a system susceptible to degrading conditions due to increasing water temperature. It is also, a significant contributor of migratory Westslope Cutthroat Trout Oncorhynchus clarkii lewisi (WCT) in the Clark- Fork Pend Oreille Basin (CFPOR). The goal of our study was to determine which tributaries to the lower Priest River are producing the most migratory WCT by locating their most likely stream of origin by genetic assignment. We expanded onto an existing genetic baseline of the CFPOR by collecting an additional 8 reference populations for fine scale assignments. Using a bi-allelic 191 loci single nucleotide polymorphism panel, we were able to assign 142 migratory WCT from Priest River and Lake Pend Oreille to their natal stream based on allele frequency. All population assignments were completed using the program RUBIAS. Primary assignment tests to reporting unit indicated approximately 82% of Priest River migrants and 35% of the Lake Pend Oreille migrants assigned back to Priest River. Of these WCT (n=70), we observed secondary assignments to 7 of the 12 tributary collections in the Priest River baseline. Based on majority Type text here high confidence posterior assignment probability scores of ≥ 0.9 (90%), we were able to determine the highest migratory producing streams as Big Creek (44.9%), Sanborn Creek (14.5%), Quartz Creek (13%), and Saddler Creek (11.6%). Our results strongly suggest these tributaries, particularly Big Creek are strongholds for WCT exhibiting a migratory life history form, and account for a large proportion of migratory WCT throughout the CFPOR. Implications for the persistence of migratory fish in this system highlight the importance of these populations. To advance our knowledge of migratory life history in Westslope Cutthroat Trout, it is essential for subsequent studies to focus on the biotic and abiotic factors influencing their expression.