Abstract
Organisms are exposed to mercury (Hg) through a variety of pathways in the environment. This study assessed two pathways of exposure for neonatal and juvenile blacktip sharks (Carcharhinus limbatus): maternal offloading and diet-based mercury exposure. Muscle, liver, kidney, and blood mercury concentrations were analyzed for neonate (n=57) and juvenile (n=13) blacktip sharks in Charlotte Harbor, an area known to contain biota with high levels of mercury. Neonates had highest concentrations in the kidney (0.559 ± 0.26 mg kg-1; n=38), followed by muscle (0.531 ± 0.17 mg kg-1; n=57), liver (0.309 ± 0.11 mg kg-1; n=38), and blood (0.0494 ± 0.033 mg kg-1; n=57). Juveniles exhibited a different distribution: liver (0.868 ± 0.54 mg kg-1; n=6), followed by the muscle (0.836 ± 0.28 mg kg-1; n=13), kidney (0.550 ± 0.22 mg kg-1; n=6), and blood (0.111 ± 0.040 mg kg-1; n=11). Based on patterns of total mercury (THg) distribution within age class and liver-to-muscle [THg] ratios, it was determined that neonate [THg] primarily originated from maternal offloading, and juveniles continued to accumulate post-parturition through dietary exposure. This study also evaluated altered behavior caused by mercury toxicity using a timed behavioral assay (n=21), but altered behavior was not apparent. Lastly, toxicity reference values derived from the literature were used to characterize and estimate risk based on exposure and potential behavioral effects evaluated. The percent likelihood that neonates had whole body concentrations exceeding the lower and upper thresholds for potential behavioral effects (37 and 1.8%, respectively) and the percent likelihood of juveniles exceeding the lower and upper thresholds (77 and 38%, respectively) were indicative of juveniles being potentially at risk for behavioral effects; however, neonates are thought to be more sensitive to toxicants than juveniles. This study contributes to the collective understanding of biomagnification in apex predators and will hopefully stimulate research of sharks in other regions to corroborate this study’s use as a baseline risk assessment for elasmobranchs.