US Geological Survey


Date of this Version



Published in U.S. Geological Survey Open-File Report 2005-1321, 1-22, (2005)


Sites of historical gold and mercury mining in the Trinity River watershed continue to release mercury to downstream water bodies. To evaluate the extent of mercury (Hg) contamination in the watershed, the U.S. Geological Survey collected samples of sediment, water, invertebrates, amphibians, and fishes from select water bodies and mine sites in Trinity County, California. This report presents total mercury data for 368 fishes collected during 2000–2002, from 4 locations within Trinity Lake, from 16 stream sites, and from 3 pond sites within the Trinity River watershed. The following species of fish were sampled (scientific name and number of samples in parentheses): brook trout (Salvelinus fontinalis, 13), brown bullhead (Ameiurus nebulosus, 5), green sunfish (Lepomis cyanellus, 13), largemouth bass (Micropterus salmoides, 33), marbled sculpin (Cottus klamathensis, 24), rainbow trout (Oncorhynchus mykiss, 237), smallmouth bass (Micropterus dolomieu, 41), and white catfish (Ameiurus catus, 2). Total mercury in 74 black bass (largemouth and smallmouth bass; Micropterus spp.) samples ranged from 0.046 to 1.225 micrograms per gram (equivalent to parts per million or ppm) wet weight (ww). Mercury concentrations in 26 of the 34 black bass (76 percent) of “legal catch size” (≥ 305 millimeters in length) were ≥ 0.3 ppm (ww), the U.S. Environmental Protection Agency water-quality criterion for the protection of human health Mercury concentrations exceeded 1.0 ppm (ww), the Food and Drug Administration action level for commercial fish in 3 of the 34 black bass (9 percent) of legal catch size. In contrast, only 3 of the 237 (about 1 percent) rainbow trout of all sizes sampled from stream, pond, and lake sites had Hg concentrations ≥ 0.3 ppm (ww). These results indicate that some fish species inhabiting select water bodies of Trinity County contain undesirably high concentrations of mercury in their skinless fillets. In response to data generated by this study and other related investigations, the California Environmental Protection Agency’s Office of Environmental Health Hazard Assessment (OEHHA) issued a draft fish-consumption advisory report that offered guidelines for human consumption of fish. The final version of the OEHHA fish-consumption advisory was approved by the State of California in July 2005 and is scheduled for publication in October 2005 (