Comparison of Adult Steelhead and Resident Trout Collected at Roza Dam in 2006 to Available DNA-based Microsatellite Genetic Data

Scott M. Blankenship, Cheryl A. Dean, and Maureen P. Small

Washington Department of Fish and Wildlife, Molecular Genetics Laboratory

600 Capitol Way N, Olympia, WA 98501-1091

Steelhead, the anadromous form of Oncorhynchus mykiss, occupying the Yakima River are a part of the Inland Steelhead – Middle Columbia River Evolutionary Significant Unit (ESU). Yakima steelhead stocks are of special interest in this ESU since natural production is declining and stock abundance is thought to be roughly 1% of its pre-1890 level. Small et al. (2006) extensively examined the population genetic structure of steelhead collected from tributaries in the Yakima River basin including: Satus Creek, Toppenish Creek, Ahtanum Creek, Naches River, and the upper Yakima River (at Roza Dam). Small et al. (2006) reported significant genetic differences among collections from most Yakima River tributaries, although collections from Roza Dam and Naches River were genetically similar, suggesting recent common ancestry and/or high gene flow. The present study extends the analysis of Small et al. (2006), and has three objectives: 1) compare a 2006 collection from Roza Dam to the previous collections from the upper Yakima, 2) determine if collections from Roza Dam constitute a population mixture, and 3) identify if N=6 resident trout collected above Roza Dam are different genetically from the steelhead collections from Roza Dam. We compared allele frequency distributions between the 2006 Roza Dam collection and previously reported data from Small et al. (2006). The genetic variation observed for the 2006 Roza Dam collection was consistent with all other Roza Dam collections. To assess whether the Roza Dam collections were mixtures of distinct populations above Roza Dam, we tested for allele associations within loci (i.e., Hardy-Weinberg Equilibrium) and among loci (i.e., Linkage Disequilibrium) in the five replicated collections from Roza Dam. The 2000 and 2001 Roza Dam collections were consistent with HWE expectations, and the 2003, 2005 and 2006 had slight deviations from equilibrium expectations. Roza Dam collections showed minimal linkage disequilibrium. These results suggest that Roza Dam collections are not composed of a mixture of populations upstream of the Dam. Regarding the resident trout samples, neither observed genetic diversity nor individual assignment methods were able to distinguish the trout from the steelhead collections from Roza Dam, suggesting they are genetically similar to upper Yakima River steelhead.