Pacific lamprey Entosphenus tridentatus are declining in the Columbia River Basin and larval lamprey use of large, mainstem river habitats is unknown. Their use of shallow depositional areas associated with tributary inputs is equally unknown. We used a deepwater electrofisher to explore occupancy, detection, and habitat use of larval Pacific lamprey and Lampetra spp. in the lower reaches and mouths of the Klickitat, White Salmon, and Wind rivers, tributaries to Bonneville Reservoir and the Columbia River. We repeated similar work conducted in 2011. Specifically, sampling in 2011 in the White Salmon River and mouth took place prior to the breach of Condit Dam and subsequent release of sediments from Northwestern Reservoir. The Wind River and Klickitat River were used as reference rivers. We used a generalized randomized tessellation stratified (GRTS) approach to select sampling quadrats in a random, spatially-balanced order and used a deepwater electrofisher to collect larval lamprey. Pacific lamprey, Lampetra spp., and unidentified lamprey occupied all strata. We calculated reach-specific detection probabilities which ranged from 0.03 to 0.29. Detection was lowest at the White Salmon River mouth and lower Klickitat River and highest at the Wind River mouth. A newly-formed delta is now present at the White Salmon River mouth and is occupied by Pacific lamprey; this habitat did not exist prior to the breach and removal of Condit Dam. The effect of water level management in these shallow habitats on larval lamprey should be considered when conserving these important species.
