Genetic Characterization of Oncorhynchus mykiss Prior to Dam Removal with Implications for Recolonization of the Elwha River Watershed, Washington

Abstract

For more than 100 years, two dams blocked upstream migration of steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) on the Elwha River, Washington. Prior to the removal of both dams (completed in 2015), 30 spatiotemporal collections of resident Rainbow Trout, steelhead, hatchery steelhead, and hatchery-derived Rainbow Trout (1,949 individuals) were made from 17 sites in the river, and the pattern of genetic diversity and connectivity were evaluated using 13 microsatellite loci. Wild-origin steelhead spawned below the downstream dam and were genetically distinguishable from upriver (above dam) resident Rainbow Trout (F ST = 0.034), and the resident Rainbow Trout segregated into two distinct groups (F ST = 0.056). Nonnative-origin hatchery steelhead varied from the indigenous steelhead (F ST = 0.029), and the hatchery trout differed from the resident trout (F ST = 0.163). Collections of resident Rainbow Trout from the upper portion of the basin were distinguished by lower estimates of genetic variability (H e , A R , and A/L) and effective population size compared with resident Rainbow Trout in the middle reaches of the Elwha River. The break between the two trout groups coincided with Rica Canyon, 8 river kilometers upstream from the Glines Canyon Dam (the upstream dam), suggesting that the upper and middle trout groups represent historic O. mykiss groups separated by flow conditions in the canyon prior to dam construction. Anticipating the potential for genetic exchange between steelhead and resident Rainbow Trout following dam removal, we evaluated the ability of the microsatellite baseline to distinguish F 1 crosses between the life history groups with computer simulations. These results demonstrate how a genetic baseline can be used as a conservation management tool to measure potential genetic introgression among resident populations and recolonizing anadromous populations.

Ancestry and Adaptation of Rainbow Trout in Yosemite National Park

“…Winans et al. ), but we did not find the complete replacement of indigenous ancestry that has been observed in some regions of California subjected to intensive hatchery trout stocking (e.g., southern California; Abadía‐Cardoso et al. ).…”

“…Within the UTR and UMR, many populations show at least some evidence of mixed ancestry, as is common in studies of O. mykiss above dams (e.g. Winans et al 2017), but we did not find the complete replacement of indigenous ancestry that has been observed in some regions of California subjected to intensive hatchery trout stocking (e.g., southern California; Abadía-Cardoso et al 2016). Although admixed populations do not represent pure indigenous lineages, they often have high genetic diversity and should not be entirely discounted when considering source populations for recovery efforts .…”

“…Importantly, despite the dramatic difference in osmotic conditions between reservoirs and the ocean, selection for an adfluvial migratory life history appears to affect the same adaptive genomic variants on Omy5 as true anadromous migrations (Pearse et al 2014;Leitwein et al 2017). This suggests that adfluvial Rainbow Trout populations isolated above dams and reservoirs could potentially contribute to the recovery of migratory anadromous ecotypes once migratory access to the ocean is restored through dam removal or assisted fish passage (Thrower et al 2008;Meek et al 2014;Winans et al 2017).…”

“…Thus, in considering efforts to reconnect migratory steelhead populations below dams with their historical upriver spawning habitats, an important first step is to evaluate the genetic ancestry and adaptive potential of the Rainbow Trout trapped above the dams (Winans et al. , , ).…”

“…Importantly, only the anadromous (steelhead) life history is listed under the ESA, whereas even closely related above-barrier Rainbow Trout populations are not protected by the ESA (NMFS 2006). Thus, in considering efforts to reconnect migratory steelhead populations below dams with their historical upriver spawning habitats, an important first step is to evaluate the genetic ancestry and adaptive potential of the Rainbow Trout trapped above the dams (Winans et al 2010(Winans et al , 2017(Winans et al , 2018.…”