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Robust Recolonization of Pacific Lamprey Following Dam Removals

Feb 8, 2021


Removal of two dams in the Elwha River basin, Washington, started one of the largest river restoration projects ever attempted in the Pacific Northwest. These dams had eliminated Pacific Lamprey Entosphenus tridentatus populations upstream. After the dam removals, larval production increased in the upper watershed, but the sources and numbers of new adult spawners were unknown. We applied genetic stock identification (GSI), parentage assignment (PA), and sibship assignment (SA) methods to (1) determine the origins of Pacific Lamprey larvae and juveniles, (2) quantify the increase in numbers of successful Elwha River spawners (i.e., effective number of breeders [Nb]) and assess whether the current numbers of spawners have reached levels equivalent to those of neighboring undammed basins, and (3) determine the relative productivity of streams within the Elwha River and how overall productivity originating from this system may be distributed across the broader surrounding region. We utilized a highly accurate set of 263 single‐nucleotide polymorphism (SNP) loci to perform PA and SA (>99% accuracy rate) and an additional set of 28 SNPs for GSI. Our results showed that a single stream (Indian Creek) was the source of 41% of larval and juvenile production in the Elwha River. Our Nb estimates for the Elwha River indicated a 12‐fold increase in Nb during the 3 years after dam removal, with recent Nb estimates matching those of neighboring Olympic Peninsula basins. These results indicate rapid recolonization potential for this highly dispersive species, and high productivity within the Elwha River suggests that restoring passage to adequate habitat is a highly effective approach for re‐establishing populations of Pacific Lamprey in coastal systems.


Jon Hess, Rebecca Paradis, Mary Moser, Laurie Weitkamp, Thomas Delomas, and Shawn Narum


Hess, J.E., R.L. Paradis, M.L. Moser, L.A. Weitkamp, T.A. Delomas, and S.R. Narum. 2021. Robust recolonization of Pacific lamprey following dam removals. Transactions of the American Fisheries Society 150(1):56-74. Online at



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Journal Article