Bioaccumulation and Maternal Transfer of Mercury in Sharks off the Southeastern United States and in the Northern Gulf of Mexico
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Mercury (Hg) is a global, pervasive, nonessential trace element that is capable of biomagnifying through marine food webs, bioaccumulating in tissues of large predatory species, and being maternally transferred during embryonic development. Since many shark species occupy high trophic positions, they are at increased risk of Hg exposure through their diet. To better understand Hg concentrations in sharks off the southeastern United States and in the northern Gulf of Mexico, total Hg (THg) concentrations were measured in the muscle, liver, and fin of ten species [spinner shark, Carcharhinus brevipinna; bull shark, Carcharhinus leucas; blacktip shark, Carcharhinus limbatus; sandbar shark, Carcharhinus plumbeus; tiger shark, Galeocerdo cuvier; lemon shark, Negaprion brevirostris; Atlantic sharpnose shark, Rhizoprionodon terraenovae; scalloped hammerhead, Sphyrna lewini; great hammerhead, Sphyrna mokarran; and smooth hammerhead, Sphyrna zygaena] using a direct mercury analyzer. Maternal transfer of Hg in embryonic sharks was also investigated by measuring THg concentrations in the muscle, heart, brain, kidney, liver, skin, and fin of bull, blacktip, and sandbar shark embryos. In the adult sharks, both inter- and intraspecies differences in THg concentrations were observed. Overall, the greatest mean THg concentrations (µg/g dry weight) were measured in the liver, followed by the muscle, and fin, with the smooth hammerhead, lemon shark, and blacktip shark containing the greatest mean liver concentrations (98.0, 61.1, and 44.0 µg/g dry weight, respectively) while the smooth, scalloped, and great hammerhead shark contained the greatest mean concentrations in the muscle (53.2, 13.8, and 11.6 µg/g dry weight, respectively). Significant positive relationships between muscle and liver THg concentration and body length were also observed for bull, blacktip, and sandbar sharks. Except for the Atlantic sharpnose shark, the mean muscle THg concentration for each species exceeded the U.S. Food and Drug Administration (FDA) 1 µg/g wet weight action limit for human consumption, indicating that sharks should be consumed infrequently to limit the risk of Hg exposure in humans. The greatest mean THg concentrations in the bull, blacktip, and sandbar shark embryos were measured in the muscle followed by either the heart or kidney, and the lowest mean concentrations were measured in the fin and liver. Overall, concentrations of THg were greater in bull and blacktip embryos than in sandbar embryos. When compared to the parent muscle and liver THg concentrations, the muscle of these embryos contained a greater percentage of maternal THg than the liver, despite the greater THg concentrations observed in adult liver, with the highest percentages being measured in the blacktip embryos, followed by bull and sandbar embryos. The results of this study suggest that THg is accumulating to high concentrations in the tissues of sharks caught off the southeastern United States and in the northern Gulf of Mexico, and therefore the consumption of these species should be limited to reduce the risk of Hg exposure within humans. In addition, the accumulation of THg in embryos provided further evidence to suggest that maternal transfer represents a significant exposure pathway for THg in sharks that could, in part, account for the elevated THg tissue concentrations previously observed in young-of-year sharks.