Hydropower dams in the lower Columbia River may contribute to declines in the populations of anadromous Pacific lamprey Lampetra tridentata by limiting access to historical spawning locations. To identify obstacles to migration, we documented the movements of radiotagged adult Pacific lampreys in specific areas of fishways (entrances, collection channels, transition areas, ladders, and counting stations) at the first three dams they encounter as they move upstream (Bonneville, The Dalles, and John Day). From 1997 to 2000, 147-299 radio-tagged lampreys were released downstream from Bonneville Dam. In 1997 and 2000, we also moved 50 radio-tagged lampreys each year to positions upstream from Bonneville Dam to assess the passage success of fish that had not passed through an entire fishway (i.e., ''naive fish''). The passage efficiency of lampreys at Bonneville Dam was 38-47%, and the median time required to pass over the dam ranged from 4.4 to 5.7 d. In contrast, 50-82% of the lampreys passed over The Dalles Dam in each year, and passage times ranged from 2.0 to 4.0 d. Passage efficiency was lowest at John Day Dam, but that estimate was based on relatively few fish. After entering the fishways, lampreys had the greatest difficulty (1) negotiating collection channels and transition areas that lacked attachment sites and (2) passing through the Bonneville Dam counting stations. Unexpectedly high passage success was documented in the ladders, where maximum current velocities could exceed 2.4 m/s. We found no evidence that lampreys released downstream from Bonneville Dam had higher passage success at The Dalles Dam than naive fish. In each year up to 60% of the lampreys made multiple entrances at the fishways, indicating that lampreys persistently attempted to pass upstream. Dams in the lower Columbia River impede adult Pacific lamprey migration, and only 3% of the fish we tagged reached areas above John Day Dam.

The Pacific Lamprey Entosphenus tridentatus is an anadromous fish that has severely declined in the Snake River basin. In 2007, the Nez Perce Tribe initiated translocation of adults to this region from the main-stem Columbia River and has continued the translocations to this day. These actions are aimed at restoring larval abundance along with holistic habitat improvements. We performed parentage and sibship analyses with 260 single-nucleotide polymorphism loci to monitor productivity of translocated lamprey over a decade (2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014)(2015)(2016)(2017)(2018). These results provide the first direct evidence that translocations boosted larval abundance, increased juvenile production in the interior Columbia River (~3% in 2017 and 2018), and demonstrated successful migration to the Pacific Ocean. Per capita juvenile production from Snake River adult lamprey translocations out-performed that of their volitionally migrating counterparts in the interior Columbia River, demonstrating that translocating adults to suitable habitats increased overall productivity. We projected that the translocations may eventually return enough adult offspring to the Columbia River to replace the annual take of adults for translocations, despite many adults likely returning to other basins. These translocations are also restoring Pacific Lamprey to historical areas where they had been nearly extirpated, which has impactful benefits both culturally and ecologically. Further, this study provided an unprecedented volume of data that has redefined key biological attributes, including timing of life stage transformation (average of 6.7-year-old juveniles), larval growth rates (~22 mm/year for the first 4 years), ocean duration (average of 5.1 years), dispersal (~3% of Snake River-origin adults that return to the Columbia River will divert to the Willamette River), and life span (median age of postspawn adults was 12.9 years). We even quantified a new biological phenomenon of delayed spawning, which occurs at low rates (~9%) in nature. These data provide an opportunity to monitor ongoing translocation efforts and inform adaptive management to aid species recovery.