Pacific lamprey Entosphenus tridentatus are declining in the Columbia River Basin and larval lamprey use of large, mainstem river habitats is understudied. Information on their use of shallow depositional areas associated with tributary inputs in the mainstem and non-tributary shallow water areas is equally lacking. We used a unique deepwater electrofisher to explore occupancy, detection, and habitat use of larval Pacific lamprey and Lampetra spp. in Bonneville and The Dalles pools and associated river mouths of the Hood, Klickitat, Little White Salmon, White Salmon, Wind, and Deschutes rivers, as well as shallow-water pool margins in the Bonneville Pool. We used a generalized randomized tessellation stratified (GRTS) approach to select sampling quadrats in a random, spatially-balanced order. Hydrodynamic modeling techniques were used to delineate the shallow water strata in Bonneville Pool. Pacific lamprey and Lampetra spp. occupied all strata. We calculated reach-specific detection probabilities which ranged from 0.00 to 0.18. Detection was lowest in the The Dalles Pool and highest at the mouth of the Wind River in the Bonneville Pool. Detection rates were relatively high in tributary mouth areas indicating the importance of this habitat for larval rearing. A newly-formed delta is now present at the White Salmon River mouth (Bonneville Pool) 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 and potential stranding in these shallow habitats on larval lamprey should be considered when conserving these important species.
