This study was conducted during September-October 2002 to verify preliminary findings of elevated total mercury concentrations in skinless fillets of sportfishes inhabiting Lake Natoma. Although we measured total mercury concentrations, most mercury in fish flesh occurs in the methylated form. In August 2000, other investigators collected a small number of fish containing mercury concentrations that exceeded 0.30 mu g/g wet weight, the U.S. Environmental Protection Agency(USEPA) tissue residue criterion derived from a reference dose for methylmercury that may cause undesirable neurological abnormalities in human infants exposed in utero when pregnant women consume mercury-contaminated foods. During our study, skinless fillets of bluegill, Lepomis macrochirus, contained as much as 0.19 mu g Hg/g wet weight (1.06 mu g Hg/g dry weight); redear sunfish, L. microlophus, contained as much as 0.39 mu g Hg/g wet weight (1.99 mu g Hg/ g dry weight); and largemouth bass, Micropterus salmoides, contained as much as 0.86 mu g Hg/g wet weight (3.85 mu g Hg/g dry weight). Maximum concentrations of mercury in other fish species varied from 0.097 mu g/g wet weight (0.537 mu g/g dry weight) in rainbow trout, Oncorhynchus mykiss, to 0.56 mu g/g wet weight (3.07 mu g/g dry weight) in white catfish, Ameiurus catus. Altogether, 1 of 20 redear sunfish, 14 of 61 largemouth bass, I of 1 brown bullhead, A. nebulosus, 2 of 3 spotted bass, M. punctulatus, and 1 of 1 white catfish exceeded the USEPA fish tissue methylmercury residue criterion. Only bluegill and largemouth bass exhibited significant correlations between fish total length (TL), weight, and age, and total mercury concentration in fillets. Judging from a best-fit power-curve equation, largemouth bass measuring 273 mm TL (roughly 292 g) or larger are estimated to contain total mercury concentrations in their fillets that exceed the USEPA fish tissue methylmercury criterion. These results confirmed that some fish species inhabiting Lake Natoma are contaminated with undesirably high concentrations of mercury in their skinless fillets.
