Estimation of occupancy, density, and abundance of larval lampreys in tributary river mouths upstream of dams on the Columbia River, Washington and Oregon

We estimated occupancy, density, and abundance of larval Pacific lamprey (Entosphenus tridentatus) and Lampetra spp. in tributary river mouths to impounded portions of the Columbia River, Washington and Oregon, using count data from deepwater electrofishing. Count data were analyzed by Bayesian methods using zero-inflated N-mixture models modified to include our experimentally derived estimate of capture probability of 0.70 (95% CI: 0.63-0.77). Lampetra spp. were only collected in river mouths in Bonneville Reservoir, while Pacific lamprey were also captured from river mouths in The Dalles and John Day reservoirs. In occupied river mouths, mean densities were commonly 0.2-0.3.m(-2), but ranged from 0.18 to 1.72.m(-2) for Pacific lamprey and 0.24 to 1.72.m(-2) for Lampetra spp. Although there was spatial overlap, estimated density peaked in the Klickitat River mouth (556 600 larvae) for Pacific lamprey and in the Wind River mouth (544 800 larvae) for Lampetra spp. Our study demonstrates considerable larval rearing in river mouths to impounded portions of the Columbia River; however, information on survival is needed to evaluate the contribution of this production to population growth and conservation.