The Influence of Pleistocene Glaciations on the Genetic Population Structure of Bull Trout in the Skagit River, WA

The long-term persistence of threatened bull trout populations requires management actions that are, in part, designed to conserve genetic diversity that is reflective of evolutionary processes.  A lack of information regarding population connectivity or historical demography of a threatened species may result in mismanagement. We used DNA from bull trout populations throughout the Skagit River, WA and Fraser River, BC to explore the unique demographic history and contemporary genetic population structure of bull trout in the Skagit River. Major biogeographical patterns of bull trout distribution in the U.S. and Canada occurred during the post-Wisonsinan glacial period between 10,000 and 20,000 years ago. During this period, bull trout colonized large proglacial lakes formed during the retreat of the Cordilleran Ice Sheet in areas of the Skagit River. Our study examines the evolutionary history of bull trout populations in this region to determine their likely origin. Our results suggest that bull trout from the Upper Skagit River were likely founded by Fraser River populations prior to the breach of the Skagit divide, which resulted in the capture of the upper Skagit drainage by the lower Skagit River. These results provide biological evidence which support the geomorphological history of the region.