Dietary mercury exposure to endangered California Clapper Rails in San Francisco Bay

•We report total mercury (Hg) in the diets of endangered California Clapper Rails.•Spatial variation in dietary Hg matched patterns described previously in Clapper Rail blood.•Invasive eastern mudsnails posed the highest Hg risk to Clapper Rails.•Local concentrations of Hg in potential prey items can provide a useful surrogate for Hg risk to Clapper Rails.

California Clapper Rails (Rallus longirostris obsoletus) are an endangered waterbird that forage in tidal-marsh habitats that pose risks from mercury exposure. We analyzed total mercury (Hg) in six macro-invertebrate and one fish species representing Clapper Rail diets from four tidal-marshes in San Francisco Bay, California. Mercury concentrations among individual taxa ranged from lowest at Colma Creek (mean range: 0.09-0.2 μg/g dw) to highest at Cogswell (0.2-0.7), Laumeister (0.2-0.9) and Arrowhead Marshes (0.3-1.9). These spatial patterns for Hg matched patterns reported previously in Clapper Rail blood from the same four marshes. Over 25% of eastern mudsnails (Ilyanassa obsolete) and staghorn sculpin (Leptocottus armatus) exceeded dietary Hg concentrations (ww) often associated with avian reproductive impairment. Our results indicate that Hg concentrations vary considerably among tidal-marshes and diet taxa, and Hg concentrations of prey may provide an appropriate proxy for relative exposure risk for Clapper Rails.