Physical, Chemical, and Biological Factors that Contribute to the Variability of Mercury Concentrations in Largemouth Bass Micropterus salmoides from Missouri Reservoirs.

Large-bodied predatory sportfish from Missouri reservoirs can contain elevated methylmercury concentrations that are of concern to the health of consumers. The concentration of total mercury (tHg) in the muscle (which > 95% is in the methylated-Hg form) of harvestable-sized largemouth bass (Micropterus salmoides; LMB) was examined to determine which factors contributed to the variability of tHg concentration in sportfish populations among Missouri reservoirs. Mean tHg concentrations in LMB from each reservoir were compared to physical and chemical characteristics of the reservoir and to biological attributes of each LMB population. Low concentrations of tHg (70-170 ng/g wet weight) in LMB from large reservoirs (surface area ≥ 35,680 acres) were likely related to the dilution of chemical Hg forms with water volume and depth. The highest tHg concentrations in LMB (268-542 ng/g) were from reservoirs with low particulate inorganic material (< 1.5 mg/L) and chlorophyll a concentrations (< 14.6 μg/L), and from LMB populations with a low proportion of large fish (proportional size distribution of LMB > 12 inches was < 33%). These relationships suggest that resource competition among LMB likely contributed to tHg bioaccumulation in reservoirs < 930 acres. Small reservoirs located in northern Missouri also may have greater methylation potential due to warmer water temperatures and anoxic conditions, but more data are needed to confirm these interactions. Fish consumption advisories for reservoirs with large surface area and volume could be reduced from one fish meal per month to one per week. To improve Missouri fisheries and protect consumers, management strategies to limit methylation and improve fish growth should be considered to reduce methylmercury bioaccumulation in small- and medium-sized reservoirs.