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Mercury in fish and fish-eating birds, with special reference to the Pinchi Lake region of British Columbia, Canada Weech, Shari Ann

Abstract

Previous studies have shown that fish-eating wildlife risk elevated methylmercury (meHg) exposure in environments where Hg concentrations, chemical speciation, and/or water chemistry favor Hg methylation and accumulation in fish. Prior to this study, however, it was not known whether Hg from a natural geologic source or contamination resulting from Hg-mining activities would bioaccumulate through the aquatic food chain to concentrations critical to fish-eating bird survival and reproduction. To investigate this, fish and fish-eating birds from five lakes in close proximity to the Pinchi fault in central British Columbia, an area of elevated natural geologic Hg and Hg mining contamination (Pinchi Lake), were studied. In addition to Hg analyses, selenium (Se) concentrations in muscle, age, length, and trophic position were used to possibly explain variations in Hg concentrations in rainbow trout {Oncorhynchus mykiss) and northern pikeminnow (Ptychocheilus oregonensis). Results showed fish Hg concentrations were positively related to age and length, and in northern pikeminnow specifically, Hg concentrations were positively related to trophic position as measured using stable nitrogen isotopes (δ⁵¹ 5N; n = 42, P < 0.0001) and negatively related to Se concentrations (n = 46, P = 0.014). Mercury concentrations in red-necked grebe (Podiceps grisgena) eggs (n = 24), and in blood and feathers of adult bald eagles (Haliaeetus leucocephalus; n = \3) and eaglets (n = 43) were also determined. All grebe eggs, including those from Pinchi Lake (n = 6), were below 0.5 pg/g Hg wet weight, often cited as the lowest observed adverse effect level for Hg developmental toxicity in birds. Mercury concentrations in the blood and feathers of eaglets, including those from Pinchi Lake (n = 12, from 8 nests), were similar or lower than previous studies conducted in areas of non-point source Hg contamination. Analysis of Hg concentrations of one-inch subsections along the length of 12 adult eagle secondary feathers from 12 individual birds also showed a significant trend of decreasing Hg concentrations from the tip to the base of the feather (P < 0.001 to P = 0.022). Eagles were also monitored for reproductive success and productivity during the summers of 2000, 2001 and 2002 (n = 13 to 15 breeding pairs annually). Over the three seasons, bald eagle reproductive success (the total number of active territories found at the beginning of May that produced 8-week-old eaglets) was similar at Pinchi Lake compared with all other study lakes combined [8/13 vs. 18/28, respectively, P = 0.95]. Average productivity (the total number of chicks produced per active territory) over the three-year study was 0.98 at Pinchi Lake (n = 12 chicks) compared to 1.17 on all other study lakes combined (n = 32 chicks, P = 0.483). These values are also comparable to reproductive success and productivity of bald eagles from reference regions outside the study area. Based on these results, increased dietary Hg exposure due to proximity to a Hg mining and natural geologic Hg source, does not appear to result in critically elevated Hg concentrations in fish-eating birds nor does it adversely affect bald eagle reproductive success. An additional study of 82 bald eagles found dead or dying in other areas of British Columbia (1987 to 1994) showed evidence of elevated Hg exposure and possible poisoning of bald eagles. Post-mortem examinations were conducted on all eagles and livers were analyzed for total Hg content, as well as meHg and Se in 17 individuals. In total, 67 eagles were classed as "low exposure", 14 eagles were classed as "moderate exposure", and one eagle was classed as "high exposure" to Hg. This latter individual was judged to have likely died of Hg poisoning, with a liver Hg content of 130.3 μg/g dry weight, of which approximately 77% was meHg. The 17 bald eagles examined for meHg and Se in their livers had a higher percentage of liver total Hg present as meHg compared to other bird species with similar total Hg concentrations in the liver. In addition, the molar ratio of Hg to Se in these bald eagle livers was higher compared to other bird species. These two factors may make eagles more vulnerable to the toxic effects of meHg compared to other bird species, although further research would be required to confirm their susceptibility. A potential genetic biomarker of meHg exposure was also investigated using livers from adult ring doves (Streptopelia risoria; n = 40) and juvenile common loons (Gavia immer; n = 31). This was the first study to examine both adult and immature avian species for DNA breakage in response to dietary meHg exposure. Results indicated that chronic consumption of diets containing environmentally relevant concentrations of meHg does not result in a significantly increased incidence of liver DNA strand breakage in dosed birds compared to controls. Therefore, it does not appear that measurement of DNA breakage would be a useful biomarker of meHg exposure in wild birds at this point.

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