Mercury Concentration in Fish from Streams and Rivers Throughout the Western United States

We collected and analyzed 2,707 large fish from 626 stream/river sites in 12 western U.S. states using a probability design to assess the regional distribution of whole fish mercury (Hg) concentrations. Large (>120 mm total length) fish Hg levels were strongly related to both fish length and trophic guild. All large fish that we sampled exceeded the wet weight detection limit of 0.0024 μg·g-1, and the mean Hg concentration in piscivores (0.260 μg·g-1) was nearly three times that of nonpiscivores (0.090 μg·g-1). Fish tissue Hg levels were not related to local site disturbance class. After partialing out the effects of fish length, correlations between Hg and environmental variables were low (r < 0.3) for the most common genera (trout and suckers). Stronger partial correlations with Hg (r > 0.5) were observed in other genera for pH, stream size, and human population density but patterns were not consistent across genera. Salmonids, the most common family, were observed in an estimated 125,000 km of stream length, exceeded 0.1 μg Hg·g-1 (deemed protective for fish-eating mammals) in 11% of the assessed stream length, and exceeded the filet equivalent of 0.3 μg Hg·g-1 (USEPA tissue-based water quality criterion) in 2.3% of that length. Piscivores were less widespread (31,400 km), but they exceeded the 0.1 and 0.3 μg Hg·g-1 criteria in 93% and 57% of their assessed stream length, respectively. Our findings suggest that atmospheric transport is a key factor relative to Hg in fish across the western United States.