Lessons from the Reintroduction of a Noncharismatic, Migratory Fish: Pacific Lamprey in the Upper Umatilla River, Oregon
David A. Close, Kenneth P. Currens, Aaron Jackson, Andrew J. Wildbill, Josh Hansen, Preston Bronson, and Kimmo Aronsuu
Abstract.—Between 1999 and 2007, more than 2,600 adult Pacific lampreys Entosphenus tridentatus (formerly Lampetra tridentata) were reintroduced to the Umatilla River, where they had been extirpated by poisoning, from nearby locations in the Columbia River consistent with the International Union for Conservation of Nature and Natural Resources guidelines. Reintroduced adult Pacific lampreys were able to find suitable spawning habitat, construct nests, and deposit viable eggs (81–93% mean egg viability per nest). Their larvae were able to feed and grow. Median lengths for age 0+, 1+, and 2+ larvae were 19, 63, and 109 mm, respectively. Mean density of larvae in survey plots increased over time from 0.08 to 6.56 larvae/m2. Geographical distribution of larvae in the river increased downstream, but larvae failed to become established in the lower Umatilla River where water flows were regulated for irrigation. Annual abundances of trapped, recently metamorphosed, out-migrating larvae increased during the study from nearly zero to 180,000, but not in all years, which suggests that many might not be surviving migration to the Columbia River, possibly because of irrigation withdrawals. Abundances of trapped, returning adult lamprey also increased from 2003 to 2006, which corresponded with the period when adult lampreys that were the progeny of reintroduced lampreys were expected to return, but long-term monitoring is necessary to confirm that increases were the result of the reintroduction. Our results also demonstrated that even if presumptive causes of extirpation were known and removed before reintroduction, success is not guaranteed. Reintroduction not only assists in redistributing animals to parts of their historical range, but in conjunction with monitoring, it may be essential to identify additional limiting factors that were unknown at reintroduction.