Trophic Ecology and Environmental Conditions Affect Mercury Concentrations in Immature Sharks in Texas Bays

Date

2023-12

Authors

Rodriguez, Jasmine

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Abstract

Sharks are ecologically important predators and are among the most threatened marine fishes due to over-fishing, climate change, and exposure to contaminants. Because many shark species are higher trophic-level consumers, they are prone to exposure to bioaccumulative and biomagnifying contaminants such mercury (Hg) through their diet. However, there is limited data on Hg concentrations in young-of-the-year and juvenile sharks (hereafter, immature) occupying coastal embayments and how Hg in immature sharks along coastlines are affected by spatial variation in diet and environmental conditions. I measured Hg concentrations in immature individuals of three shark species [blacktip shark (Carcharhinus limbatus), bull shark (Carcharhinus leucas), and bonnethead shark (Sphyrna tiburo)] in four Texas bays: Lower Laguna Madre, Corpus Christi Bay, Aransas Bay, and Sabine Lake, which represent a range of abiotic conditions and environmental settings. Shark dorsal muscle tissue and putative prey were assessed for Hg and stable isotopes (δ13C, δ15N, and δ34S) to determine how inter- and intraspecies variation in trophic ecology influenced Hg exposure risk. I also additionally assessed how spatial variation (inter-bay differences) in environmental conditions (salinity, water temperature, and dissolved oxygen) may influence overall Hg concentrations in sharks and their prey. There were interspecies differences in immature shark Hg, with bull sharks generally having higher Hg across sites. Dietary mixing models and estimates of niche overlap derived from stable isotopes indicated that bull sharks across sites derived a greater proportion of their diet from sciaenid fishes [e.g., red drum (Sciaenops ocellatus) and spotted seatrout (Cynoscion nebulosus)], which generally had greater Hg concentrations than other potential prey items. In contrast, bonnethead sharks showed a greater dietary contribution of crustaceans [e.g., blue crab (Callinectes sapidus) and brown shrimp (Farfantepenaeus aztecus)] and blacktip sharks had a greater contribution of Gulf menhaden (Brevoortia patronus) and brief squid (Lolliguncula brevis). These results indicate that trophic ecology of immature sharks can exhibit substantial interspecies variation, which influences Hg exposure risk. In addition, I found that Sabine Lake and Aransas Bay had greater Hg concentrations in immature sharks than other bays, but estimates of Hg trophic magnification slopes (i.e., the rate of Hg accumulation with trophic position) did not differ among bays. However, Hg concentrations in a baseline organism found in all bays [eastern oyster (Crassostrea virginica)] exhibited greater Hg concentrations in Sabine Lake and Aransas Bay. I hypothesize that spatial differences in conditions which influence microbial Hg methylation, bioavailability, and inputs (i.e., salinity and dominant watershed vegetation type) contributed to the observed biota Hg differences among bays. This study is one of the first to examine the relationship between diet, environmental conditions, and Hg in immature sharks in Texas bays and makes a substantial contribution to understanding the ecotoxicology of Hg in elasmobranchs.

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Keywords

sharks, mercury, trophic ecology, salinity, Texas, estuary, biomagnification

Citation

Rodriguez, J. (2023). Trophic ecology and environmental conditions affect mercury concentrations in immature sharks in Texas bays (Unpublished thesis). Texas State University, San Marcos, Texas.

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