Pre-spawning migration of adult Pacific lamprey, Entosphenus tridentatus , in the Willamette River, Oregon, U.S.A.
We describe the migration distances and timing of the adult Pacific lamprey, Entosphenus tridentatus , in the Willamette River Basin (Oregon, U.S.A.). We conducted aerial surveys to track radio-tagged fish upstream of a major waterfall and hydropower complex en route to spawning areas. We detected 24 out of the 43 fish that passed the waterfall-hydropower complex. Of the detected fish, 17 were detected multiple times. Their maximum migration distance upstream in the mainstem Willamette approximated a normal distribution. The maximum distance migrated upstream did not significantly correlate with total body length ( r  = −0.186, P  = 0.385) or date that the fish passed Willamette Falls ( r  = −0.118, P  = 0.582). Fish migrated primarily during the spring to early summer period before stopping during the summer, when peak river temperatures (≥20°C). However, at least three fish continued to migrate upstream after September. Behavior ranged from relatively slow migration, followed by holding; to rapid migration, followed by slow migration further up in the basin. This study provides a basis for informing more detailed research on Pacific lamprey in the future.