Mercury in Fish in North Mississippi Reservoirs: Statistical Analysis and Risk Assessment

Mercury (Hg) is a pollutant of particular concern because it is wide-spread, persistent, and poses a serious risk to human health. Once released into the atmosphere, it cycles through the environment by a series of complex biogeochemical processes. Upon deposition to aquatic ecosystems, inorganic mercury can be transformed to methylmercury by microorganisms. Methylmercury is a known human neurotoxin that can bioacculmulate in fish tissue and biomagnify up aquatic food chains. Consumption of contaminated fish is the primary route of human exposure to methylmercury. The Mississippi Department of Health has issued fish consumption advisories for Grenada and Enid Lakes in the Yazoo River Basin as a result of elevated mercury concentrations. This study involved a statistical analysis of mercury data for Crappie (CR), Largemouth Bass (LMB), and Channel Catfish (CC) from Grenada, Sardis, and Enid Lakes in Northern Mississippi; total Hg concentrations were compared between Lakes and between species. A mercury risk assessment for consumption of fish from the lakes was also conducted using different assumption variables to evaluate the effectiveness of the existing fish consumption advisories. Linear regression analysis of length vs. weight suggested that LMB and CC exhibited similar growth trends regardless of lake. The relationship between length and weight for CR from Enid Lake was statistically different from that of CR from Grenada and Sardis, suggesting that environmental factors unique to Enid Lake may affect the growth of CR there. Of the fish analyzed, LMB consistently had the highest mean v mercury concentrations (mean ± SE, n = 14 – 20, Grenada: 630 ± 104 ng/g, Sardis: 334 ± 40 ng/g, Enid: 386 ± 36 ng/g), followed by CC (Grenada: 437 ± 42 ng/g, Sardis: 222 ± 21 ng/g, Enid: 152 ± 14 ng/g) and then CR (Grenada: 199 ± 17, Sardis: 147 ± 8 ng/g, Enid: 214 ± 10 ng/g). Even taking length into account, Grenada Lake produced the LMB and CC with the highest Hg concentrations, suggesting the Hg concentration may be higher there than at Sardis and Enid. Only LMB had a strong relationship between length and Hg concentration by linear regression analysis. Because the existing fish consumption advisories are length-based, the lack of relationship between length and Hg concentration means the recommendations may be insufficient to protect the public from exposure to MeHg. Seven different risk assessment paradigms yielded hazard index (HI) and monthly consumption limit (MCL) values that further discredit the existing consumption advisories and many consumption recommendations. An HI greater than one is indicative of an individual’s risk of toxicity associated with exposure to a toxicant, here methylmercury. LMB from Grenada had an adult HI>1 by all seven risk calculations. Similarly, all fish species from all three lakes yielded HI>1 for children