Insights, From Genetic Analyses, Into Stock-Specific Distribution Of Juvenile Steelhead (Oncorhynchus Mykiss) From The Columbia River During Early Marine Migration

For anadromous Pacific salmon (Oncorhynchus sp.), ocean conditions during their initial entry into the marine environment can greatly affect their survival. Different life history types or stocks may experience different conditions during their marine entry because routes of early marine migration can differ among types or stocks. Steelhead (O. mykiss) from the Columbia River are believed to migrate offshore quickly once they enter the ocean, but little is known about whether life history or stock-specific differences in early marine migration exist. We assembled a baseline of steelhead genetic data that allowed us to estimate the genetic stock of origin for juvenile steelhead that had been caught off the coasts of Washington and Oregon in May, shortly after their out-migration from freshwater. We found differences in the average locations of the various genetic stock groups of the Columbia River, dissimilarities that were most likely due to differences in the timing of the marine entry of juveniles. We also observed considerable variation among years in the average location where we caught steelhead and in the number of steelhead caught, results indicating that freshwater or marine conditions can influence the behavior or survival of steelhead.