Mercury contamination and toxicity threaten Pacific Lamprey recovery in the Columbia basin. Measured methylmercury concentrations in larval Pacific Lamprey from tributaries in the mid-Columbia were as high as 4.0 µg/g wet weight.1 At these concentration levels, injury is predicted to occur in more than 50% of larval and adult fish and more than 90% of early-life stage fish.2 These high levels of mercury contamination may cause significant harm to larval/juvenile fish, including direct mortality, failure to hatch, developmental abnormalities, and reproductive harm. There is a need to understand mercury sources and concentrations in larval lamprey to address the water quality threats identified in the Mid-Columbia 2021 Regional Implementation Plan. This study will use recent advancements in mercury isotope analysis to identify the sources of mercury accumulating in larval/juvenile Pacific Lamprey from their native rearing habitat. Mercury isotope ratios can be used to identify sources of mercury by comparing to common sources (‘end-members’). Common end-members include atmospheric deposition, mining, industrial, and soil. The Yakama Nation Fisheries will collect larval lamprey and sediment samples from Columbia River tributaries for total mercury, methylmercury, and isotopic (mercury, carbon, and nitrogen) analyses. USGS will use mercury isotope ratios to identify mercury sources3 and document if there have been shifts in tissue concentrations since the 2014 study. If analysis identifies non-atmospheric sources in tributary streams, it suggests that managers could use restoration efforts to mitigate mercury contamination. Chemistry results will be published in the U.S. Geological Survey’s ScienceBase data repository and interpreted in a peer-reviewed manuscript.