Migratory Behavior and Genetic Diversity Patterns of Bull Trout in the Skagit River, WA

We investigated the migratory behavior of Skagit River bull trout using acoustic telemetry to better understand the migratory behavior, population and genetic structure, and life history diversity of this threatened species.  The Skagit River is located in 3,200 sq-mile watershed in Washington State that possesses one of the largest bull trout populations in the United States.  Bull trout were implanted with acoustic tags within major basins and habitat areas in the Skagit Watershed, including the upper and lower river, freshwater delta, and estuary.  Tagging was initiated in 2003 and continued through 2008, with over a hundred bull trout tagged during the study.  We tracked the tags using an array 30 hydrophones deployed throughout the Skagit River watershed and within the Skagit estuary, and an array of over 100 hydrophones deployed along the marine nearshore areas of the Puget Sound.  Bull trout in the Skagit were found to have highly diverse migratory and life history pattern, including anadromous, fluvial, and resident life history forms.  Anadromous life history types originated from all accessable areas of the watershed, but were the dominant life history types in the glacial-dominated Sauk and Suiattle river subbasin.  Fluvial forms were dominant in upper Skagit River subbasin, including areas below hydroelectric dams.  Most anadromous bull trout spend extended periods (one to two years) in marine nearshore and estuary areas of the Puget Sound, and often migrate into other river systems in the northern and central sound prior to returning to their natal spawning streams in the Skagit.  Alternatively, fluvial bull trout reside mainly in areas of the mainstem river within a few miles of there natal tributaries. Bull trout were found to be genetically distinct at both the subbasin and tributary levels.  The findings of this study suggest that bull trout are capable of possessing high levels of life history variability, behavioral plasticity, and genetic diversity within the same watershed.