Density Estimates of Larval Lamprey in the Mainstem Columbia River

Lamprey are in decline throughout their native ranges and information on abundance is lacking and needed for conservation planning.  We estimated densities of larval Pacific Lamprey Entosphenus tridentatus and Lampetra spp. in the Columbia River basin upstream of Bonneville Dam.  A deepwater electrofisher was used to sample multiple sites in tributary river mouths and pools of Bonneville and The Dalles reservoirs.  Density and abundance estimates were made using modified N-mixture models evaluated using Bayesian methods.  Estimates were similar in magnitude for the two species.  Density estimates for Pacific Lamprey ranged from 0 per m² in The Dalles Pool to 0.74 per m² in the Klickitat River mouth.  Densities for Lampetra were lowest in both pools (0 per m²) and highest in the Wind River mouth (0.35 per m²).  Estimates of abundance for river mouths were often fairly high (i.e., some >100,000 individuals), but imprecise likely due to the patchy spatial distribution of larval lamprey burrowed in sediments.  Larval lamprey can be successfully collected using a deepwater electrofisher.  Estimates from N-mixture models should be considered with caution due to the patchy distribution of burrowed larvae, but could potentially be useful for evaluating local trends in abundance to aid conservation.