Occupancy and Detection of Larval Pacific Lampreys and Lampetra spp. in a Large River: the Lower Willamette River

Pacific lampreys Entosphenus tridentatus (formerly Lampetra tridentata) are declining in the Columbia River basin, and the use of large, main-stem river habitats by larvae of this species is unknown. We used a deepwater electrofisher to explore occupancy, detection, and habitat use of larval Pacific lampreys and larval Lampetra spp. in the lower Willamette River, Oregon. We used a generalized random tessellation stratified approach to select sampling quadrats (30 x 30 m) in a random, spatially balanced order. Pacific lampreys, Lampetra spp., and unidentified lampreys were found in the Willamette River; larvae were detected in all areas except the Multnomah Channel. We calculated reach- and quadrat-specific detection probabilities and the amount of sampling effort required for 80% confidence that larval lampreys were in fact absent when they were not detected. Lampreys were detected in a variety of areas (although relatively low numbers were collected), including shallow, nearshore areas; midchannel areas (depth up to 16 m); and anthropogenically affected areas. Detection probabilities (i.e., in occupied areas) were 0.07 (reach) and 0.23 (quadrat). The sampling effort required for 80% confidence that lampreys were absent when undetected was 20 quadrats (in the lower Willamette River reach) and 6 subquadrats (within a quadrat). Differences in lamprey detection by depth were not observed. A variety of sizes was collected (20-144mmtotal length), indicating the likely occurrence of multiple ages of larvae. Our study identifies how the occurrence of larval Pacific lampreys can be quantified with statistical rigor in a large river (i.e., larger than fourth order [1: 100,000 scale]). The effect of channel management activities on larval lampreys should be considered in efforts to conserve these important species.