Montana Fish, Wildlife, and Parks (MT FWP), in collaboration with the United States Geological Survey (USGS) Wyoming-Montana Water Science Center (WY-MT WSC), Montana Department of Environmental Quality (MDEQ), and the United States Environmental Protection Agency (USEPA) collected fish from the Koocanusa Reservoir in 2021 for tissue analysis. Fish tissue collected included muscle, eggs, liver, and whole body. Analysis of tissues included characterization of the concentration of selenium, mercury and methyl mercury, nitrogen and carbon stable isotopes, and vitamins A and E. The purpose of these analyses was to inform state and federal management agencies on transboundary coal mining impacts to downstream fish health.