Abstract
As apex, long-lived predators, sharks are susceptible to methylmercury (MeHg) exposure through biomagnification from prey items. Accordingly, tissue biopsy samples were collected from bonnethead (Sphyrna tiburo) and bull sharks (Carcharhinus leucas) from three estuaries within the Ten Thousand Islands National Wildlife Refuge (TTINWR) located in southwest Florida for determination of total mercury (THg). Muscle THg concentrations in bonnethead and bull sharks ranged up to 1.40 mg kg-1 and 2.60 mg kg-1 (wet-weight basis), respectively. Concentration of THg generally increased with shark size, a proxy of its age, which explained much of the intraspecific variation. Interspecific variation in THg concentrations was evident, with bull sharks accumulating higher concentrations than bonnethead sharks, despite being younger and faster growing. The higher tissue concentration was likely due to greater maternal offloading of Hg to bull shark pups, as well as differences in diet among the two species. Baseline levels of MeHg entering the food web were assessed by examining THg concentrations in oysters (Crassostrea virginica), as primary consumers within each estuary. Concentrations of THg ranged up to 0.04 in oysters and differed among bays. These results confirm continued high Hg levels in sharks of the TTINWR as compared to conspecifics from other areas in Florida. Concentrations of THg observed in these sharks may pose a threat to the health and fitness of both shark and human populations.
