Mercury Contamination and Its Relation to Trophic Ecology and Anthropogenic Pollution in Coastal and Deep Sea Shark Communities

The effect of changing anthropogenic mercury emissions on marine wildlife is of broad interest. Methylmercury can cause reproductive and neurological damage and biomagnifies in food webs. Mercury availability in the Pacific Ocean has been increasing and therefore could be expected to increase in marine fishes. In Hawaii, tunas have shown increases proportional to increasing oceanic mercury over several decades. Historical data on shark muscle mercury is also available for Hawaii, presenting an opportunity for comparison in a long-lived upper trophic level cartilaginous fish. Muscle samples were opportunistically collected from sandbar sharks in 2003-2005 for mercury analysis and comparison with published data from 1971. Mercury contamination was similar in sandbar sharks collected in the two time periods, in contrast to tunas. Sandbar sharks collected in 2003-2005 had a higher observed rate of stillborn embryos than previously documented. Since mercury can cause reproductive toxicity, muscle mercury contamination was quantified in available sandbar shark embryos. Contamination was similar in stillborn and viable embryos, indicating that mercury toxicity was likely not the cause of embryo death. Mercury does not appear to have changed in sandbar sharks and the observed frequency of stillborn sharks does not appear to be due to increasing mercury emissions over three decades. Deep sea chondricthyans comprise nearly half of global chondrichthyan fauna but have been researched relatively less than their coastal and pelagic counterparts. As long-lived mesopredators and apex predators in their food webs, deep sea shark can bioaccumulate high levels of mercury in their tissues. Mercury was measured in six species of relatively abundant deep sea sharks in the GOM that inhabit the continental shelf edge to the continental slope. To attempt to address whether the Deepwater Horizon oil spill indirectly affected bioavailability of methylmercury in GOM, the same or closely related species were also analyzed from regions not affected by the oil spill. Overall, Mustelus sinusmexicanus had the lowest mercury among the GOM sharks analyzed, and Squalus clarkae had the highest. The remaining four species were similar to one another in their mercury contamination. Regional comparisons revealed similar contamination between M. canis canis in the GOM and M. canis insularis in Eleuthera. Eleuthera S. cubensis had similar mercury contamination to GOM S. clarkae, to which they were closer in size than GOM S. cubensis. GOM Centrophorus granulosus had higher mercury contamination than Virginia C. granulosus but this was only marginally significant. There was a slight and non-significant increase in mercury with depth in the GOM. M. canis caught on the east side of De Soto Canyon had significantly higher mercury than those caught on the west side of De Soto Canyon. The opposite pattern was seen in C. uyato, with higher mercury concentrations on the west side of the canyon, but these results may be strongly influenced by sample size for both species. Since mercury bioaccumulation in organisms is tightly linked to biomagnification, mercury studies are often coupled with trophic ecology studies using light stable isotopes as tracers. Since animals vary in the quantity of lipids that they store in their tissues, and since the presence of lipids can affect δ¹³C signatures, it is necessary to explore these effects to correctly interpret δ¹³C results. A particular challenge for chondrichthyan fishes is that they store isotopically light nitrogenous wastes in their tissues for osmoregulation, and this can affect the interpretation of δ15N, sometimes leading to inaccurate interpretation of trophic level. The effects of lipids and nitrogenous wastes on stable isotope analysis of chondrichthyan fishes has been an important aspect of trophic ecology research on this group. The effects of lipid extraction, urea extraction, and combined lipid and urea extraction compared to bulk tissue were tested on a coastal and deep sea teleost and a coastal and deep sea elasmobranch as representatives that might be included in coast to deep sea food web studies. Since mercury analysis requires the same initial muscle tissue handling steps as stable isotope analysis (SIA), the effects of treatments on mercury quantification were also tested. The results suggest that chemical lipid extraction is necessary for accurate interpretation of SIA results in coastal and deep sea teleosts and elasmobranchs and that urea was extracted sufficiently during the lipid extraction, therefore additional urea extraction steps are not necessary. Additionally, it may be possible for researchers to use leftover treated muscle from SIA to obtain accurate mercury results. Future research on trophic ecology including SIA informed by this research may help to provide context for mercury findings in GOM deep sea sharks.