As apex predators, blacktip sharks (Carcharhinus limbatus) are highly susceptible to biomagnified mercury (Hg) particularly in the Gulf of Mexico (GOM), which is known to contain fishes and invertebrates with elevated Hg levels. Blacktip sharks occur in the GOM year-round and are heavily fished both commercially and recreationally, but little is known about how Hg affects the species. In this study, blood, muscle, liver, and kidney samples were collected from neonatal (n = 57) and juvenile (n = 13) blacktip sharks in Charlotte Harbor, Florida. Hg concentrations in neonates and juveniles were found to be elevated in muscle (mean ± SE = 0.59 ± 0.23 mg kg−1), liver (0.39 ± 0.29 mg kg−1), kidney (0.56 ± 0.25 mg kg−1), and blood (0.059 ± 0.041 mg kg−1) compared to other local shark species and conspecifics from other areas. Blood plasma chemistry, hematology, and liver histology were evaluated to assess the relationship between Hg and tissue damage. Plasma chemistry parameters alanine aminotransferase (ALT) and phosphorus (PHOS) were not correlated with tissue Hg or liver condition index. Hematological effects were also not correlated with tissue Hg. However, melanomacrophage presence and lipid deposition, evaluated as part of histopathological analysis, were found to exhibit a statistically significant association with concentrations of Hg in tissue and ALT levels. These results suggest that Hg exposure potentially had a negative effect on the livers of the blacktip sharks evaluated in the present study.
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We would like to thank all the FGCU students that provided field support on this project including: J. Hemberger, M. Scroggin, E. Krueger, V. Miller, T. Stiehl, J. Forbes, and J. Knoer. We would also like to thank Drs. G. Tolley and J. Douglass for their advice on early drafts of this paper and especially Drs. C. Campbell and L. Arnett Chinn for their guidance on procedures and analysis used in this project, along with an anonymous reviewer whose comments improved this manuscript. Finally, we would like to thank C. Reistad and D. Norris for their assistance and support through the sampling, analysis, and writing processes. Funding for this work was provided by an internal grant from Florida Gulf Coast University.
This study was supported entirely by funds internal to the university.
All three authors participated in sample collection and processing; Ms. Reistad and Mrs. Norris were each responsible for different analytical procedures, data analysis and interpretations (i.e., different thesis). Dr. Rumbold provided mentorship.
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The authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.
These collections were made under Florida Fish and Wildlife Conservation Commission Special Activity License SAL -15-1347A-SRP and FGCU’s Institutional Animal Care and Use Committee (IACUC) Protocol #1516-10 titled “Effects of mercury on the health of Blacktip shark embryos, neonates and juveniles”.
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Norris, S.B., Reistad, N.A. & Rumbold, D.G. Mercury in neonatal and juvenile blacktip sharks (Carcharhinus limbatus). Part II: Effects assessment. Ecotoxicology 30, 311–322 (2021). https://doi.org/10.1007/s10646-020-02325-x