Passage Efficiency of Adult Pacific Lampreys at Hydropower Dams on the Lower Columbia River, USA

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 radio‐tagged 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.