The Impacts of Dam Construction and Removal on the Genetics of Recovering Steelhead (Oncorhynchus mykiss) Populations across the Elwha River Watershed

Abstract

Dam construction and longitudinal river habitat fragmentation disrupt important life histories and movement of aquatic species. This is especially true for Oncorhynchus mykiss that exhibits both migratory (steelhead) and non-migratory (resident rainbow) forms. While the negative effects of dams on salmonids have been extensively documented, few studies have had the opportunity to compare population genetic diversity and structure prior to and following dam removal. Here we examine the impacts of the removal of two dams on the Elwha River on the population genetics of O. mykiss. Genetic data were produced from >1200 samples collected prior to dam removal from both life history forms, and post-dam removal from steelhead. We identified three genetic clusters prior to dam removal primarily explained by isolation due to dams and natural barriers. Following dam removal, genetic structure decreased and admixture increased. Despite large O. mykiss population declines after dam construction, we did not detect shifts in population genetic diversity or allele frequencies of loci putatively involved in migratory phenotypic variation. Steelhead descendants from formerly below and above dammed populations recolonized the river rapidly after dam removal, suggesting that dam construction did not significantly reduce genetic diversity underlying O. mykiss life history strategies. These results have significant evolutionary implications for the conservation of migratory adaptive potential in O. mykiss populations above current anthropogenic barriers.

Reconnecting the Elwha River: Spatial Patterns of Fish Response to Dam Removal

“…Juvenile Coho Salmon and Chinook Salmon were also seen upstream of each dam, demonstrating that spawning adults and their progeny are utilizing upstream habitats. The reconnection of the Elwha River will have far-reaching implications for genetic diversity, life history diversity, and habitat use of these species, which has started to be documented for Steelhead (Fraik et al, 2021), Pacific Lamprey (Hess et al, 2021), Coho Salmon (Liermann et al, 2017), andBull Trout (Quinn et al, 2017;Lincoln et al, 2018;Brenkman et al, 2019). Although our study was not designed to estimate population sizes, the observed densities increased and there was an upstream shift in the locations of higher density reaches for both trout and Bull Trout.…”

“…Thus, the potential emigration from upstream Rainbow Trout may have been much greater after dam removal. This emigration may have contributed to the apparent rapid recovery of Summer Steelhead in the Elwha and is supported by the observation of emigrating smolts after dam removal with genetic ancestry from populations upstream of the former dams (Fraik et al, 2021). In addition, Summer Steelhead appear to be favoring habitats within the boundaries of Olympic National Park.…”

“…These taxa are not as dependent on connectivity to complete their life-cycle as their anadromous counterparts. Previous genetic studies of Steelhead and resident Rainbow Trout found that Rainbow Trout populations were genetically segregated between ME and UE populations and that these differences likely existed prior to dam construction (Winans et al, 2017;Fraik et al, 2021). With the increase in the more fecund Summer Steelhead in the upper Elwha and the potential for interactions between resident and anadromous forms of O. mykiss , admixture of Steelhead and Rainbow Trout could have contributed to increased fitness and population sizes of resident fish (Ohms et al, 2014).…”

“…A significant number of fish migrated up to 60 km into the upper reaches of the Elwha River. Their presence and remarkable increase in abundance are likely due to numerous factors, including the contribution from resident Rainbow Trout in the middle and upper river Kendall et al, 2015;Fraik et al, 2021). Resident Rainbow Trout produced an estimated 350-700 emigrating fish that were fully capable of saltwater migration in the early 1990s (Hiss and Wunderlich, 1994).…”

Barrier mitigation measures trigger the rapid recovery of genetic connectivity in five freshwater fish species

“…If molecular tools are now commonly considered for the a priori assessment of barrier effects (Abernethy et al, 2013; Coleman et al, 2018; Dehais et al, 2010; Gouskov et al, 2016; Liu et al, 2020; Meldgaard et al, 2003; Prunier et al, 2018; Raeymaekers et al, 2009), there is still a surprising paucity of genetic studies dedicated to the temporal monitoring of changes in connectivity after restoration (Ding et al, 2019). Only a handful of recent studies could be identified, all focusing on the effect of the creation (Liu et al, 2020; Vega-Retter et al, 2020) or the removal (Fraik et al, 2021) of large dams on gene flow (but see Weigel et al, 2013). As a result, and despite the importance for practitioners and managers to assess (and communicate) the relevance of their actions for natural ecosystems, there are still very few indications that (i) current mitigation measures (barrier removal or fish pass creation) deployed worldwide are improving genetic connectivity ( sensu Lowe & Allendorf, 2010) and (ii) that molecular approaches are efficient and operational tools to prioritize local restoration actions and assess their efficiency.…”