Landlocked Pacific Lamprey Entosphenus tridentatus in the Middle Fork Willamette River, Oregon

Pacific Lamprey Entosphenus tridentatus is an anadromous fish native to the west coast of North America and the Pacific coast of Asia. In the 1950s, construction of dams on the Middle Fork Willamette River blocked upstream migration of adult Pacific Lamprey which resulted in their extirpation from otherwise suitable habitats. We report evidence of landlocked Pacific Lamprey surviving for >40 years after the construction of two high-head flood control dams (Dexter and Lookout Point) in the Middle Fork Willamette River 355.0 and 360.3 km upstream of the Pacific Ocean, respectively. In 2001, six putative Pacific Lamprey (two juveniles and four larvae) were captured using an electro shocker during a stream survey, 21.7 km upstream of Lookout Point Dam (the uppermost of these dams) and were entered into the Smithsonian National Museum of Natural History's Division of Fishes collection. We re-examined these specimens, and their morphometric measurements and dentition are consistent with Pacific Lamprey. Additional evidence supports the existence of a landlocked population of Pacific Lamprey that resided upstream of these two impassable dams for approximately five generations. To our knowledge, this is the first report of Pacific Lamprey surviving for multiple generations upstream of impassable impoundments. However, no Pacific Lamprey have been detected in recent targeted surveys, indicating that this population has since been extirpated or exists in very low numbers.