Spatio‐temporal distribution of adult Pacific lamprey Entosphenus tridentatus relative to habitat fragmentation

Pacific lamprey (Entosphenus tridentatus), a fish species native to the Pacific Northwest (USA), have distinctive cultural and ecological value but determining their spatial and temporal distribution is challenging due to a general lack systematic monitoring. In this study, we used counts of Pacific lamprey redds to model the probability of occurrence and abundance of Pacific lamprey based on environmental covariates including artificial barriers, assuming higher predicted lamprey redds translates to more suitable spawning habitats. Using generalized linear mixed zero‐inflated models, results suggest that Pacific lamprey abundance was generally lower in high gradient streams, further from the ocean. Stream reaches with warmer spring water temperatures and greater historical median spring flows supported higher abundances. Lamprey occurrence was primarily influenced by spring water temperatures and distance from the ocean. We further observed that when streams warm beyond 18°C, confidence intervals around the abundance estimates widen and zero‐inflation increases, indicating a decrease in occurrence. One objective of the study was to recommend where barrier removal or restoration should be prioritized to increase passage and thus access to upstream habitats. We considered artificial barriers to primarily influence the probability of occurrence through access. The barrier variable in this model had a negative effect on the probability of lamprey occurrence, but it was not a strong predictor in the model. While we are not able to suggest specific locations that would most benefit barrier removal or improvement based on these model results, we can identify the watersheds with a higher probability to support Pacific lamprey and provide potential additional habitats by improving habitat connectivity. Focusing restoration and/ or removal of barriers on watersheds in the Mid‐South region of the Oregon Coast (i.e., Alsea, Siuslaw, Coos, Coquille, and Sixes rivers) with higher habitat suitability could prioritize use of limited funds, increase the probability of benefiting Pacific lamprey, and potentially other native lampreys and migratory (e.g., salmon, steelhead; Oncorhynchus) species. Although this manuscript focuses on the Oregon Coast region, the methods are transferrable to other regions where Pacific lamprey are present.