Liver, kidney, and breast muscle tissue from common loons (Gavia immer) and common mergansers (Mergus merganser) from eastern Canada were analyzed for total Hg, organic Hg, and Se concentrations. In both species, Hg and Se concentrations were generally highest in liver, followed by kidney, then breast muscle. As total Hg concentrations increased in liver and kidney tissues, the fraction that was methylmercury (MeHg) decreased. Livers and kidneys with the highest total Hg concentrations (> 100 μg/g dry weight) had only 5 to 7% of the total as MeHg. Concentrations of MeHg were <10 μg\g (dry weight) regardless of the total Hg concentration in liver and kidney. In contrast, the proportion of MeHg in breast muscle remained high (80–100%) in both loons and mergansers regardless of total Hg concentration, which ranged from 0.7 to 35 μg\ (dry weight7rpar;. Strong positive correlations were observed between total Hg and Se concentrations, but not between MeHg and Se, in kidney and liver tissues for both loons and mergansers. Mercury concentrations were significantly higher (analysis of variance, p < 0.0001) in all three tissues of loons found in an emaciated condition compared with apparently healthy loons. Non‐emaciated loons had Hg concentrations comparable to those of healthy mergansers, and concentrations were lower than those associated with Hg toxicity in birds. The elevated Hg concentrations in emaciated loons may have contributed to their impaired conditions; alternatively, the wasting of their muscle and other tissues may have caused apparent tissue Hg concentrations to increase. In any case, total Hg concentrations in liver and/or kidney tissue are insufficient criteria for making toxicologically relevant judgments of possible Hg toxicity. Because of the varying degrees of apparent Hg demethylation in liver, kidney, and other tissues and the association of the resulting inorganic Hg with Se in an apparently stable, toxicologically inert complex, estimates of organic Hg and Se concentrations, in addition to total Hg, in these organs are recommended.

Abstract-Liver, kidney, and breast muscle tissue from common loons (Gavia immer) and common mergansers (Mergus merganser) from eastern Canada were analyzed for total Hg, organic Hg, and Se concentrations. In both species, Hg and Se concentrations were generally highest in liver, followed by kidney, then breast muscle. As total Hg concentrations increased in liver and kidney tissues, the fraction that was methylmercury (MeHg) decreased. Livers and kidneys with the highest total Hg concentrations (Ͼ100 g/g dry weight) had only 5 to 7% of the total as MeHg. Concentrations of MeHg were Ͻ10 g/g (dry weight) regardless of the total Hg concentration in liver and kidney. In contrast, the proportion of MeHg in breast muscle remained high (80-100%) in both loons and mergansers regardless of total Hg concentration, which ranged from 0.7 to 35 g/g (dry weight). Strong positive correlations were observed between total Hg and Se concentrations, but not between MeHg and Se, in kidney and liver tissues for both loons and mergansers. Mercury concentrations were significantly higher (analysis of variance, p Ͻ 0.0001) in all three tissues of loons found in an emaciated condition compared with apparently healthy loons. Non-emaciated loons had Hg concentrations comparable to those of healthy mergansers, and concentrations were lower than those associated with Hg toxicity in birds.

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Mercury (Hg) has been increasing in some marine birds in the Canadian Arctic over the past several decades. To evaluate the potential reproductive impact of Hg exposure, eggs of two species of arctic-breeding seabirds, the thick-billed murre and arctic tern, were dosed with graded concentrations of methylmercury (MeHg) and artificially incubated in the laboratory to determine species differences in sensitivity. Based on the dose-response curves, the median lethal concentrations (LC(50)) for thick-billed murre and arctic tern embryos were 0.48 and 0.95 μg g(-1) Hg on a wet-weight (ww) basis, respectively. Compared with published LC(50) values for other avian species, the murres and terns had a medium sensitivity to MeHg exposure. LC(50) values were also calculated for the actual Hg concentration measured in the embryos, that is, the maternally-deposited Hg plus the injected MeHg dose. This increased the LC(50) values to 0.56 μg g(-1) Hg ww in the thick-billed murre and to 1.10 μg g(-1) Hg ww in the arctic tern. Although muscarinic acetylcholine and N-methyl-D-aspartic acid glutamate receptor levels have been correlated with increasing Hg concentrations in brains of adult birds, no significant associations were found in brain tissue of the murre or tern embryos. The incidence of gross external anatomical deformities was 4.3 % in the murre embryos and 3.6 % in the tern embryos. However, given that the eggs were taken from wild populations, it is unlikely that the deformities observed in this study were due to MeHg exposure alone.