Human activities over the last several centuries have transferred vast quantities of mercury (Hg) from deep geologic stores to actively cycling earth-surface reservoirs, increasing atmospheric Hg deposition worldwide. Understanding the magnitude and fate of these releases is critical to predicting how rates of atmospheric Hg deposition will respond to future emission reductions. The most recently compiled global inventories of integrated (all-time) anthropogenic Hg releases are dominated by atmospheric emissions from preindustrial gold/silver mining in the Americas. However, the geophysical evidence for such large early emissions is equivocal, because most reconstructions of past Hg-deposition have been based on lake-sediment records that cover only the industrial period (1850-present). Here we evaluate historical changes in atmospheric Hg deposition over the last millennium from a suite of lake-sediment cores collected from remote regions of the globe. Along with recent measurements of Hg in the deep ocean, these archives indicate that atmospheric Hg emissions from early mining were modest as compared to more recent industrial-era emissions. Although large quantities of Hg were used to extract New World gold and silver beginning in the 16th century, a reevaluation of historical metallurgical methods indicates that most of the Hg employed was not volatilized, but rather was immobilized in mining waste.

Recent laboratory studies have demonstrated that environmentally realistic concentrations of dietary methylmercury can impair reproduction of fish. To evaluate relations between reproductive success and biomarkers of methylmercury exposure, we fed juvenile fathead minnows (Pimephales promelas) one of three diets contaminated with methylmercury: 0.06 (control), 0.87 (low), and 3.93 (medium) microg of Hg g(-1) dry weight. At sexual maturity, fish were paired, allowed to reproduce, and then analyzed for total mercury, plasma testosterone (T), and 17beta-estradiol (E2). Diets did not affect survival or growth of fathead minnows. Methylmercury suppressed levels of T in males and E2 in females. Male fathead minnows fed the control diet had mean T concentrations 20% and 106% greater than those fed the low and medium diets; control females had mean E2 concentrations 149% and 402% greater than those fed the low and medium diets. Methylmercury also inhibited gonadal development of females; the gonadosomatic index (GSI) of females fed the medium diet was 40% less than that of females fed control or low diets. Plasma levels of T in males and E2 in females were positively related to GSI. Methylmercury reduced the reproductive success of fathead minnows. Spawning success was 32% for pairs fed the control diet, 12% for pairs fed the low diet, and 0% for pairs fed the medium diet. Pairs fed the low diet required, on average, 5 d longer to spawn a clutch of eggs than the controls. Concentrations of methylmercury fed to fathead minnows in this study are also encountered by invertivorous and piscivorous fish in some methylmercury-contaminated aquatic ecosystems. This suggests that reproduction of wild fishes may be adversely affected by methylmercury and that suppressed hormone levels may be used to indicate diminished reproduction of fish.

Threshold concentrations associated with adverse effects of dietary exposure to methylmercury (MeHg) were derived from published results of laboratory studies on a variety of fish species. Adverse effects related to mortality were uncommon, whereas adverse effects related to growth occurred only at dietary MeHg concentrations exceeding 2.5 µg g(-1) wet weight. Adverse effects on behavior of fish had a wide range of effective dietary concentrations, but generally occurred above 0.5 µg g(-1) wet weight. In contrast, effects on reproduction and other subclinical endpoints occurred at dietary concentrations that were much lower (<0.2 µg g(-1) wet wt). Field studies generally lack information on dietary MeHg exposure, yet available data indicate that comparable adverse effects have been observed in wild fish in environments corresponding to high and low MeHg contamination of food webs and are in agreement with the threshold concentrations derived here from laboratory studies. These thresholds indicate that while differences in species sensitivity to MeHg exposure appear considerable, chronic dietary exposure to low concentrations of MeHg may have significant adverse effects on wild fish populations but remain little studied compared to concentrations in mammals or birds.

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17 18Many laboratory studies have documented that mercury can be toxic to fish, but it is 19 largely unknown if mercury is toxic to fish in their natural environments. The objective of our 20 study was to investigate the toxic effects of mercury on northern pike (Esox lucius) at Isle 21 Royale, Michigan. In 124 northern pike from eight inland lakes, concentrations of total mercury 22 in skin-on fillets ranged from 0.069 to 0.622 !g/g wet wt. Concentrations of total mercury in 23 livers increased exponentially compared with concentrations in fillets, to a maximum of 3.1 !g/g 24 wet wt. Methylmercury constituted a majority of the mercury in livers with total mercury 25 concentrations <0.5 !g/g wet wt, but declined to 28-51% of the mercury in livers with total 26 mercury concentrations >0.5 !g/g wet wt. Liver color (absorbance at 400 nm) varied among 27 northern pike and was positively related to liver total mercury concentration. The pigment 28 causing variation in liver color was identified as lipofuscin, which results from lipid peroxidation 29 of membranous organelles. An analysis of covariance revealed lipofuscin accumulation was 30 primarily associated with mercury exposure, and this association obscured any normal 31 accumulation from aging. We also documented decreased lipid reserves in livers and poor 32 condition factors of northern pike with high liver total mercury concentrations. Our results 33 suggest (i) northern pike at Isle Royale are experiencing toxicity at concentrations of total 34 mercury common for northern pike and other piscivorous fish elsewhere in North America and 35(ii) liver color may be useful for indicating mercury exposure and effects in northern pike at Isle 36 Royale and possibly other aquatic ecosystems and other fish species. 37 38

Mercury is a toxic trace metal that can accumulate to levels that threaten human and environmental health. Models and empirical data suggest that humans are responsible for a great deal of the mercury actively cycling in the environment at present. Thus, one might predict that the concentration of mercury in fish should have increased dramatically since the Industrial Revolution. Evidence in support of this hypothesis has been hard to find, however, and some studies have suggested that analyses of fish show no change in mercury concentration. By compiling and re‐analyzing published reports on yellowfin tuna (Thunnus albacares) caught near Hawaii (USA) over the past half century, the authors found that the concentration of mercury in these fish currently is increasing at a rate of at least 3.8% per year. This rate of increase is consistent with a model of anthropogenic forcing on the mercury cycle in the North Pacific Ocean and suggests that fish mercury concentrations are keeping pace with current loading increases to the ocean. Future increases in mercury in yellowfin tuna and other fishes can be avoided by reductions in atmospheric mercury emissions from point sources. Environ Toxicol Chem 2015;34:931–934. © 2015 SETAC

Contamination of fish populations with methylmercury is common in the region of the Laurentian Great Lakes as a result of atmospheric deposition and methylation of inorganic mercury. Using fish mercury monitoring data from natural resource agencies and information on tissue concentrations injurious to fish, we conducted a screening-level risk assessment of mercury to sexually mature female walleye (Sander vitreus), northern pike (Esox lucius), smallmouth bass (Micropterus dolomieu), and largemouth bass (Micropterus salmoides) in the Great Lakes and in interior lakes, impoundments, and rivers of the Great Lakes region. The assessment included more than 43,000 measurements of mercury in fish from more than 2000 locations. Sexually mature female fish that exceeded threshold-effect tissue concentrations of 0.20 μg g(-1) wet weight in the whole body occurred at 8% (largemouth bass) to 43% (walleye) of sites. Fish at 3% to 18% of sites were at risk of injury and exceeded 0.30 μg g(-1) where an alteration in reproduction or survival is predicted to occur. Most fish at increased risk were from interior lakes and impoundments. In the Great Lakes, no sites had sexually mature fish that exceeded threshold-effect concentrations. Results of this screening-level assessment indicate that fish at a substantive number of locations within the Great Lakes region are potentially at risk from methylmercury contamination and would benefit from reduction in mercury concentrations.