Mercury Exposure from Domestic and Imported Estuarine and Marine Fish in the U.S. Seafood Market

Human exposure to methylmercury (MeHg) causes a variety of adverse health effects, including developmental delays in children of exposed mothers (Cohen et al. 2005) and deficits in neurocognitive function in adults (Yokoo et al. 2003). Blood MeHg concentrations in individuals are strongly correlated with the frequency and types of seafood consumed (Mahaffey et al. 2004). However, even for pregnant women, consuming seafood has a variety of health benefits when dietary MeHg intake is known to be low (e.g., Daniels et al. 2004; Mozaffarian and Rimm 2006). Regulatory agencies rely on information about how individuals are exposed to MeHg to evaluate trade-offs among health benefits from fish consumption and potential risks of MeHg exposure. In the United States, MeHg risk management takes the form of both advisories recommending limits on amounts of high-Hg fish consumed and regulations that control emissions from human sources. Assessing the effectiveness of both strategies in terms of changes in human exposure requires data on a) geographic supply regions for fish consumed by the U.S. population, and b) concentrations of Hg in fish and shellfish. Comparing the supply of fisheries products for all individuals from the commercial market (18.9 g/person/day, 2000–2002) [National Marine Fisheries Service (NMFS) 2003] to the total intake from dietary recall surveys (16.9 g/person/day, uncooked fish weight, 1994–1996–1998) [U.S. Environmental Protection Agency (EPA) 2002] shows that mean consumption estimates are comparable in magnitude. Hence, across the entire U.S. population, most seafood consumed comes from the commercial market. Estuarine and marine fish and shellfish dominate the edible supply of fish in the commercial market, comprising > 90% of the market share (Carrington et al. 2004). Thus, dietary intake of MeHg from estuarine and marine seafood accounts for most exposure in the U.S. population. Although many studies have investigated how variability in amounts and types of fish consumed affects MeHg exposure, few addressed uncertainties resulting from natural stochasticity in MeHg concentrations within seafood categories in the commercial market. Instead, most studies rely on Food and Drug Administration (FDA) survey data to characterize Hg concentration distributions (e.g., Carrington and Bolger 2002; Carrington et al. 2004; Mahaffey et al. 2004; Tran et al. 2004). However, FDA survey data are usually aggregated into one mean Hg concentration for each commercial market category. This can be problematic because each market category (e.g., fresh and frozen tuna) may describe a number of different biological species (e.g., for tuna: albacore, bigeye, bluefin, skipjack, yellowfin) with different growth rates and dietary preferences that affect Hg bioaccumulation. In addition, fish and shellfish in the commercial market consist of domestic landings from the Atlantic and Pacific oceans and imported species from a variety of countries. Many researchers have reported geographic variability in Hg concentrations among commercially important fish and shellfish species. For example, various tuna species caught in the Atlantic, Pacific, and Mediterranean oceans have significantly different length- and weight-normalized tissue Hg residues (Adams 2004; Anderson and Depledge 1997; Brooks 2004; Morrisey et al. 2004; Storelli et al. 2002). In addition, although imported shrimp make up a large fraction of domestic seafood consumption (NMFS 2003), Hg concentrations reported by the FDA are typically below detection limits (FDA 2006a, 2006b). However, measured Hg concentrations in shrimp caught in a variety of countries vary by an order of magnitude (Minganti et al. 1996; Plessi et al. 2001; Ruelas-Izunza et al. 2004). Although high Hg concentrations can sometimes be attributed to sampling at contaminated sites (Chvojka et al. 1990) or age and size classes of fish not commonly found in the commercial seafood market, Burger et al. (2005) also found significant differences between nationwide FDA values and Hg levels in fish sold in seafood markets in the New Jersey region. Based on these data, we can hypothesize that variability in Hg intakes within each species category in the commercial market is not adequately captured by grouping Hg concentrations in fish caught in geographically diverse regions into a single population mean. Better resolution in Hg concentration data used for exposure assessments may be obtained by grouping survey data by the origin of each marine and estuarine seafood product in the commercial market. This study assessed how estimated Hg exposure from estuarine and marine seafood in the U.S. population is affected by variability in Hg concentrations among different supply regions. To do this, supply of fisheries products were divided into categories based on the geographic sources of seafood in the commercial market consumed by the U.S. population. Expected Hg intake rates for different age groups, such as children and women of childbearing age, were modeled using Hg concentration data from each supply region, market share, and total consumption of each species from the NMFS (2001, 2002, 2003). Data from the U.S. Department of Agriculture’s Continuing Survey of Food Intake by Individuals (CSFII) (U.S. EPA 2002) and the National Health and Nutrition Examination Survey (NHANES) (NCHS 2006) provided information on variability in consumption patterns and body weights in the U.S. population. Distributions of intakes calculated in this study from geographically explicit Hg data were compared with values obtained using FDA Hg concentrations to assess whether variability in Hg concentrations by species and geographic regions significantly affects per capita intakes used to evaluate risks associated with Hg exposure. Geographically referenced exposure data provide a building block for quantitatively assessing how global changes in environmental Hg concentrations will affect human exposure to Hg in the United States.