Elevated mercury levels in a declining population of ivory gulls in the Canadian Arctic

Abstract

The ivory gull (Pagophila eburnea) is a high Arctic marine bird which is in serious population decline for reasons unknown. Local ecological knowledge interviews and colony surveys indicate an 80% decline in numbers of nesting ivory gulls in Arctic Canada since the early 1980s (Gilchrist and Mallory, 2005). Ivory gulls feed largely on fish and invertebrates, but also scavenge placentae, faeces and carrion of marine mammals (Haney and MacDonald, 1995). Given their relatively high trophic position in marine food webs (Hobson et al., 2002), contaminants have been proposed as one stressor which could be affecting this species. Elemental mercury (Hg) is highly volatile, and gaseous Hg partitions readily into the atmosphere where it can undergo long-range atmospheric transport. Due to a variety of factors (Macdonald et al., 2005), polar regions are global sinks for Hg. Mercury biomagnifies up the food chain (Atwell et al., 1998; Campbell et al., 2005) making those species feeding at high trophic positions more vulnerable to Hg exposure via their diet. The most bioavailable and toxic form of Hg is methylmercury and nearly 100% of the Hg transferred by breeding female birds to their eggs is methylmercury (Wiener et al., 2003). Retrospective analyses of Hg in archived seabird eggs from the Canadian Arctic have shown steady increases in concentrations over the past few decades (Braune et al., 2001). In this study, total Hg was measured in archived samples of ivory gull eggs collected from Seymour Island in the western Canadian Arctic (Fig. 1) in 1976 and 1987, in addition to eggs collected from that same location in 2004. Those data were then compared with published toxicity threshold levels to determine whether Hg may be playing a role in the population decline of this species. Ivory gull eggs were collected by hand from Seymour Island, Nunavut, Canada (Fig. 1) during 1976 (n = 9), 1987 (n = 9) and 2004 (n = 6) under appropriate Canadian Wildlife Service (CWS) collection permits. Each egg was removed from a different nest to maintain independence among samples. Eggs collected in 1987 and 2004 were kept cool in the field and shipped to the National Wildlife Research Centre (NWRC) of CWS, for processing, archival and residue analyses. Eggs collected in 1976 were preserved in 10% formalin in the field prior to shipment to NWRC where they were stored frozen prior to homogenization in the 1980s. Egg contents from all years were homogenized and stored frozen ( 40 C) in acid-rinsed polyethylene vials. In a study on the effects of preservation techniques, no Hg was found in unused 10% formalin fixative, and there was no evidence of loss of Hg into the fixative from museum fish samples preserved for a period of at least 40 years (Kelly et al., 1975). Likewise, it has been shown that preservation of tissues in 10% formalin has minimal effect on dN (Sarakinos et al., 2002). Collection details for eggs of other seabird species collected from the Canadian Arctic in 2003–2004 and presented in Table 2 can be found in Braune (2004). Individual ivory gull eggs were analyzed for total Hg at NWRC using an Advanced Mercury Analyzer (AMA-254) equipped with an ASS-254 autosampler for solid samples according to CWS Method No. MET-CHEM-AA-03E. The method employs direct combustion of the sample in an oxygen-rich atmosphere. Analytical accuracy was determined using four standard reference materials (DOLT-2, DOLT-3, and TORT-2 obtained from the Canadian National Research Council; Oyster Tissue 1566b obtained from the US National Institute of Standards and Technology) and 15 random egg samples analyzed in replicate. Recovery of reference materials was within the confidence interval of the certified values and the nominal detection limit for total Hg was 0.05 lg g 1 dry weight sample. This same method was used to analyze total Hg in eggs of the other seabird species collected from the Canadian Arctic in 2003–2004 (see Table 2) except that those eggs were analyzed as pooled (composite) samples of three eggs each. Freeze-dried homogenates for individual ivory gull egg samples were also analyzed for stable nitrogen isotopes (N/N) by the Department of Earth Sciences at the University of Ottawa. Stable-nitrogen isotope assays were performed on 1 mg subsamples of homogenized materials by loading them into tin cups and combusting them at 1,800 C in a Robo-Prep elemental analyzer. Resultant N2 gas was then analyzed using an interfaced Europa 20 :20 continuous-flow isotope ratio mass spectrometer (CFIRMS) with one laboratory standard run for every eight samples analyzed. Stable-nitrogen abundance was expressed in d notation as the deviation from standards in parts per thousand (&) according to the following equation: