Genetic Stock Identification to Evaluate the Origins of Migrating Kelt Steelhead Interrogated at Lower Granite Dam, Idaho

Anadromous steelhead trout (Oncorhynchus mykiss) are ESA-listed as threatened throughout the Snake River.  Implementing conservation and management strategies for steelhead trout can be particularly challenging owing to the complex life history of the species.  Steelhead trout maintain the potential for an iteroparous life history, including post-spawn survival, secondary migrations, and repeat spawning.  Although iteroparity provides the opportunity for individuals to spawn across multiple years, and may increase lifetime fecundity and genetic diversity, population-specific rates of iteroparity are variable.  Such variability in the incidence of secondary migration may impart local differences in productivity potential and demographic stability.  The distinct B-run steelhead typically found in the Clearwater River and Salmon River are commonly larger than A-run fish, owing to greater ocean residence time (two vs. one year, respectively).  Iteroparity has been shown to be negatively correlated with size, and a recent study identified greater proportions of kelt (post-spawn) steelhead among populations of smaller A-run steelhead trout; this included populations from the Grande Ronde and Imnaha rivers.  In our evaluation we used genetic stock identification (GSI) to examine the origins of kelt steelhead (n=1500) interrogated during downstream migration at Lower Granite Dam (LGD) in the Snake River Basin.  In cooperation with the Idaho Department of Fish and Game, we have established an extensive genotypic data baseline using a suite of 192 single nucleotide polymorphism (SNP) loci.  Our objective was to identify or resolve genetic distinctions among collections in the baseline as reporting units for GSI evaluation of our LGD mixed stock sample (unknown origin in the Snake River).  On the basis of genetic similarity among populations we have currently identified ten such reporting units including three in the Clearwater River, four in the Salmon River, a Grande Ronde unit, an Imnaha unit, and a lower Snake River unit.  The kelts sampled at LGD were assigned to reporting groups using likelihood tests.  Results of our evaluation will be used to better understand the distribution of post-spawn kelts (inferring a potential iteroparous life history) throughout the region on a temporal scale as well as the relative association of an iteroparous life history with both hatchery- and natural-origin sources.