Abstract
The biomagnification of methylmercury (MeHg) through the world’s oceans begins with absorption at the primary producer level and culminates in the highest mercury (Hg) burdens in apex predators and humans. The health concerns associated with excessive Hg consumption and accompanying toxicity are detrimental to both fish and humans. As apex predators, blacktip sharks (Carcharhinus limbatus) are highly susceptible to biomagnified Hg particularly in the Gulf of Mexico (GOM), which is known to contain fishes and invertebrates with elevated Hg levels. Blacktips occur in the GOM year-round and are a heavily fished species both commercially and recreationally, but little is known about how Hg affects the species. High Hg burdens can be mobilized by females during reproduction and transferred to the blood and tissues of developing embryos and eggs—a phenomenon referred to as maternal offloading. In this study, blood, muscle, liver, and kidney samples were collected from neonatal (n=57) and juvenile (n=13) blacktip sharks gillnetted in Pine Island Sound, Florida. Combined Hg concentrations ([Hg]) in neonates and juveniles were found to be elevated in muscle (0.588 ± 0.23 mg kg-1), liver (0.385 ± 0.29 mg kg-1), kidney (0.558 ± 0.25 mg kg-1), and blood (0.0594 ± 0.041 mg kg-1) compared to other species and conspecifics from other areas. Blood plasma chemistry, hematology, and liver histology were evaluated to assess the relationship between maternal offloading of Hg and tissue damage. Plasma chemistry parameters alanine aminotransferase (ALT) and phosphorous (PHOS) were not correlated with tissue [Hg] or liver condition index. Hematological effects (e.g., displaced erythrocyte nuclei and vacuolization of erythrocytes) were not correlated with tissue [Hg]. Several types of histopathologies were observed, but only three were statistically related to [Hg]: melanomacrophage presence x juvenile liver [Hg] (p=0.0201), melanomacrophage presence x juvenile [ALT] (p=0.0201), and adipose deposition x neonate liver [Hg] (p=0.0025). These results suggest that Hg exposure may have had some limited effects in the blacktips but that they may also be responding to other stressors.