Mercury Risk to Avian Piscivores across the Western United States and Canada

As part of the Western North America Mercury Synthesis, we evaluated risk to avian piscivores across all or parts of their breeding range through the compilation of fish and bird tissue mercury (Hg) concentrations for western United States and Canada. We used a novel approach by targeting a suite of avian piscivore species including representatives from both freshwater (Bald Eagle, Osprey, Common and Yellow-billed Loon, Western and Clark’s Grebe, Belted Kingfisher) and marine (Laysan and Black-footed Albatross, Common and Thick-billed Murre) ecosystems for assessing risk across all or a large part of a species’ breeding range. For freshwater species, we used a combination of prey fish and avian tissue Hg concentrations. For marine species, we used colony-specific egg concentrations to determine risk to Hg exposure. All samples were converted to a standard metric of bird blood equivalent to compare across geographic areas and taxa. Our assessment indicates that birds foraging on larger sized fish (likely higher trophic position) are at greatest risk to Hg exposure. We identified two biological Hg hotspots (the Great Basin of Oregon and Nevada, and the Prairie Pothole region of Alberta and Saskatchewan) where at least four of the five target freshwater breeding piscivores show clusters of grid cells at high risk. Five areas of concern are also identified where definitive data is lacking but preliminary assessments indicate elevated risk, including the Yukon River Δ, Yukon River Flats, Puget Sound, San Francisco Bay, and the desert southwest in Arizona.