Patterns in distribution and density of larval lampreys in the main-stem Columbia River, Washington-Oregon

ObjectiveThis study compiled data sets from multiple research efforts from 2010 to 2018 to describe patterns in the presence and density of larval lampreys in deep water habitats across 430 river kilometers of the lower and middle main-stem Columbia River.MethodsWe used logistic regression to evaluate the influence of landscape-level factors on Pacific Lamprey Enthosphenus tridentatus and Lampetra spp. presence. Additionally, we used N-mixture models to estimate larval lamprey densities in six river mouths in the spring and fall of multiple years.ResultAt the landscape-level, the models suggested that the probability of presence for both Pacific Lampreys and Lampetra spp. decreased with increasing distance from the ocean, distance from the closest upstream tributary river mouth, and distance from the nearest main-stem riverbank. The probability of presence also varied by upstream tributary river mouth. The probability of larval presence in river mouths was an order of magnitude greater than in reservoir pools. Evaluating river mouth habitats, larval lamprey densities varied seasonally, annually, and among river mouths, ranging from 0.04 to 9.63 larvae/m2. Results generally suggested broader distributions and higher densities within river mouths during spring when flows were high compared to the fall when flows were lower, although not in all river mouths or across all years. Larval densities increased in the Wind and Klickitat rivers (the most consistently examined tributary river mouths) over the study period.ConclusionOur findings suggest that the main-stem Columbia River supports larval lamprey rearing year-round. Probability of presence was highest and variable among river mouths, suggesting the importance of some river mouths as rearing habitats. Understanding shifts (seasonal, annual, and those as result of changes in environmental conditions) in lamprey presence and density could inform how specific management actions (e.g., dewatering for in-river work) and their timing could affect larval lampreys in rivers.