The genetic consequences of dam removal on the Elwha River for recolonizing Steelhead

Primary Author: Alexandra Fraik

Faculty Sponsor: Joanna Kelley


Primary College/Unit: Arts and Sciences

Category: Physical and Social Sciences

Campus: Pullman




Dam construction and riverscape habitat fragmentation disrupt important life-histories and movement of aquatic species. Functionally, dams disrupt the movement of water, sediments, nutrients, and species between upstream and downstream. The Elwha and Glines Canyon dams in Washington state, constructed in the early 1900s, fragmented the Elwha River, restricting migratory species such as Oncorhynchus mykiss from ocean. Oncorhynchus mykiss exhibits multiple life-history phenotypes including a migratory form (anadromous Steelhead) and a freshwater non-migratory form (resident Rainbow Trout). The negative effects of dams on O. mykiss have been extensively documented (including population declines, loss of genetic diversity, extirpation), however few have studied their population genetics following dam removal. In 2012 and 2015, the Elwha River dams were removed and anadromous Steelhead recolonized formerly dammed regions. In this study, we characterized the genetic ancestry of recolonizing Steelhead into the previously dammed portions of the Elwha River watershed. Single nucleotide polymorphism genetic data were produced from 1,003 samples collected prior to, during and post dam removal from both life-history forms. Population structure analyses supported three genetic clusters primarily explained by dam location. Following dam removal genetic structure decreased. Using genetic stock identification, we determined recolonizing Steelhead were of mixed genetic ancestry with individuals derived from formerly dammed populations as well as below dam populations. Alleles putatively involved in the anadromous phenotype were detected in formerly dammed populations both prior to and post dam removal. These results have significant evolutionary implications for the conservation of migratory adaptive potential in O. mykiss populations facing changing riverscapes.