Level and extent of mercury contamination in Oregon, USA, lotic fish

We conducted a probability survey of 154 Oregon, USA, stream and river sites to assess the spatial extent of mercury (Hg) contamination in fish tissue. Samples consisted of whole-fish analyses of both small (<120 mm) and large (>120 mm) fish at each site, when both were present. Overall, Hg levels (microgram/g) in small fish (mean = 0.031; standard deviation [SD] = 0.029), large piscivores (mean = 0.284; SD = 0.175), and large invertivores (mean = 0.055; SD = 0.047) were found within fairly narrow ranges; always above detection (0.0025 microgram/g) and almost always below 0.4 microgram/g. Given the great ecoregion diversity across Oregon, the narrow range in fish tissue Hg levels suggests that atmospheric transport is an important vehicle for Hg distribution. In small fish, Hg levels were almost always low and showed little meaningful difference among fish taxa. In large fish, Hg levels were significantly related to fish length. Piscivores (pikeminnow and bass) had significantly higher Hg levels, and the slope of their Hg level/length relationship was much steeper than for invertivores. Salmonids, the most commonly occurring fish taxon in Oregon, exceeded 0.1 microgram Hg/g (deemed protective for fish-eating mammals) in an inferred 15% of stream lengths where they occurred. Pikeminnows and bass were found at fewer sites, but they exceeded 0.1 microgram Hg/g in an inferred 96 and 70%, respectively, of stream lengths where they occurred.