Using Aquatic Environmental DNA (eDNA) to Track Fish Recolonization Following Barrier Removal: An Example from the Elwha River, Olympic National Park, Washington

Tools for evaluating the outcomes of migratory fish recolonization following barrier removal are needed. One such tool, equally applicable to native and non-native species, is the use of aquatic environmental DNA (eDNA). We developed a collection of species-specific molecular markers for use in PCR amplification for both indigenous and introduced fish species recolonizing the Elwha River after dam removal. The largest project of its kind, a key goal of the Elwha River restoration is the return of anadromous fish to protected spawning and rearing habitats upstream of the dams in the Olympic National Park wilderness. However, tracking the rate and extent of recolonization is challenging and costly, as current techniques applicable in the easily accessible frontcountry are untenable in the roadless backcountry. We targeted 11 fish species across 56 river kilometers at 10 frontcountry and 14 backcountry sites from both the Elwha River (n=14) and its tributaries (n=10) weeks after the final dam removal. We detected Chinook salmon 16 river kilometers upstream of the former Glines Canyon Dam site, farther upstream than limited visual surveys indicated. Environmental DNA appears robust, is applicable over wide spatial extents, and is a relatively low cost method to monitor taxa following barrier removal.