Mercury is a global environmental pollutant with important adverse health effects, particularly on neurodevelopment in the fetus. This relationship was first brought to light in the well known episode at Minamata Bay, Japan where pregnant women were exposed to high concentrations of methylmercury (MeHg) by eating contaminated seafood [1
]. Subsequent long term cohort studies of residents of the Faroe Islands [2
] confirmed and extended these findings to indicate that prenatal exposure to MeHg is associated with cognitive and psychomotor impairment in children, although some controversy persists due to conflicting results from other populations [3
]. Prenatal exposure to mercury can result in multiple neurodevelopmental effects including deficits in memory, language, attention and fine motor skills [4
]. Current evidence suggests that adult exposure to MeHg may also have subtle effects on the nervous, immune and cardiovascular systems, although these associations are weaker and less consistent than those associated with pre-natal exposure [7
Human exposure to MeHg is primarily from the consumption of fish and shellfish [5
]. Several studies have demonstrated an association between dietary intake of fish and the accumulation of mercury in the body [8
]. Coastal subpopulations often consume more fish than the general population [11
]. This increase is likely driven by regional access to fresh seafood [12
]. As a result, coastal human populations typically have higher concentrations of mercury in their tissues compared to inland populations [13
]. In Florida, the average adult consumes approximately 46 g per day of seafood, considerably higher than the estimated 4.5 g per day for the general population in the United States [13
]. In a survey of sport fish consumption among women of childbearing age from twelve states, women in Florida consumed more per year than those from other states [14
]. Further, environmental concentrations of mercury are generally higher than those found in other states [16
]. Therefore, there is a need for regional estimates of mercury exposure among individuals who consume seafood at a higher frequency than the general population of the US.
A variety of industrial processes release mercury into the atmosphere which is subsequently deposited in aquatic environments. Inorganic mercury in the aquatic environment is converted to MeHg through bacterial action and bioaccumulates through trophic levels of the food web [17
]. As a result, apex predators accumulate the highest tissue concentrations of mercury in the ecosystem. We became aware of a potential public health risk to humans along the Florida coastline through studies of mercury accumulation in blood and skin samples of Atlantic bottlenose dolphins (Tursiops truncatus
) inhabiting the Indian River Lagoon, FL (IRL). These analyses measured total mercury (THg) concentrations of 658 ± 519 μg/L wet weight in blood [18
] and 7.0 ± 5.9 μg/g dry weight in skin [19
]. Approximately 73 percent of THg in dolphins was MeHg [19
]. Concentrations in blood and skin were more than four times higher than those found in bottlenose dolphins sampled in Charleston Harbor, SC [18
]. Elevated mercury concentrations were associated with perturbation of multiple hepatic, renal, endocrine, and hematological parameters suggesting deleterious health effects in adult dolphins [20
]. High concentrations of mercury in skin have also been found in dolphins from Florida’s west coast [21
] and the Mediterranean Sea [22
As an apex predator, Atlantic bottlenose dolphins in the IRL have a long life span, bioaccumulate anthropogenic contaminants and have defined home ranges making them a valuable sentinel species [23
]. The high mercury concentrations found in IRL dolphins may reflect environmental differences in mercury contamination that include deposition, biogeochemistry and trophic transfer [24
]. Similarly, these factors impact the concentration of mercury found in fish species that local human populations may be consuming. Therefore, the main objective of this study was to examine exposure to mercury among coastal residents living near the IRL. The secondary objective was to examine associations between the frequency, species and sources of seafood consumed by residents and their hair mercury concentration.
To the authors’ knowledge, this is the first attempt to assess hair mercury concentrations and seafood consumption among recreational anglers and residents living along the east coast of Florida. It is also the first to apply findings from bottlenose dolphins as a stimulus to explore the potential for similar risk among humans from the same geographical region. Study participants had a mean hair mercury concentration of 1.5 µg/g. The U.S. EPA exposure guideline, which equates approximately to a hair mercury concentration of 1 µg/g [28
], was exceeded in 50% of the samples obtained from participants. The concentrations found in Florida women of all ages in this study (0.96 µg/g) were approximately five times higher than those from a randomly generated sample of US women of childbearing age (0.19 µg/g) from NHANES [29
]. In addition, the concentrations of hair mercury in this study were higher than those reported from anglers and coastal resident populations in Canada (0.82 µg/g), Wisconsin (0.86 µg/g), Alabama (0.55 µg/g) and Louisiana (1.1 µg/g) [9
Nearly 45 percent of study participants reported consuming any seafood once a week or less, a lower frequency than the FDA recommended 12 ounces (two average meals) per week of seafood containing low concentrations of mercury [31
]. However, 56 percent of those surveyed reported eating fish and shellfish three times per week or more which is higher than the recommended frequency. Participants consumed a variety of fish species. Commercial market data showed that typical US consumers derive most of their methylmercury exposure from a narrow range of fish and shellfish types, most of which were imported, not locally caught [32
]. At the national level, estimated Hg intake is associated with a few commonly consumed fish types with moderate to high concentrations of Hg, such as canned tuna (0.35 µg/g) and swordfish (0.98 µg/g) [32
]. In the current study, individuals who consumed locally caught grouper, sea trout and snapper at least once a week had significantly higher hair mercury concentrations compared to those who consumed these species less than once a week in the past three months.
Males had a significantly higher concentration of hair mercury and were 2.12 times more likely to have a total hair mercury concentration above 1µg/g compared to females. Similar differences in concentration by gender have been reported from Japan, Wisconsin and Louisiana [9
]. A probable explanation for these results is that males tend to do more recreational fishing and consumption of their catch compared to females. Previous studies have demonstrated that the main sources of seafood for females tend to be shrimp and canned tuna from grocery stores and seafood markets [34
]. The same pattern was seen in the current study. In addition, men commonly consume more fish per kg of bodyweight during a meal compared to females [9
]. As a result, males who are frequent recreational anglers represent a highly exposed subpopulation along the Indian River Lagoon.
Age was not associated with increased mercury concentration. The lack of an association with age was also reported among women of childbearing age in Florida [35
] and sportfish consumers in Canada [30
]. However, other studies have found that hair mercury concentration was associated with increased age [12
]. Native populations had the highest reported mercury concentrations in multiple studies compared to other ethnic groups [12
]. However, potential differences in mercury concentration and fish consumption patterns by race and ethnicity could not be evaluated in this study since the sample was almost exclusively caucasian.
Higher levels of educational attainment were associated with hair mercury concentration in current analyses for the total population but not in gender-stratified analyses. Education and high household income (>$75,000/year) were also associated with increased hair mercury concentration and shellfish consumption in a 12 state study of women and with awareness of state-issued consumption advisories for fish consumption [14
]. Higher socioeconomic status may be linked to increased access to recreationally acquired sportfish and the ability to purchase relatively expensive foodstuffs. In addition, persons with higher levels of education may be more aware of the health benefits associated with fish consumption.
There was a statistically significant difference in hair mercury concentration by county of residence with residents of Martin county having the highest mean hair mercury concentration (2.08 ± 2.81 µg/g) followed by Brevard county (1.88 ± 1.79 µg/g). The differences could not be explained by differences in the proportion of males in these two counties. However, a higher proportion of participants from Martin County reported consuming seafood three times a week or more compared to other counties. Both counties border directly on the IRL and have higher mean income and proportion of residents with at least a high school education compared to the general population in Florida [38
Several limitations of this study are important in interpreting the findings. Although identification of high concentrations of mercury in bottlenose dolphins led to the identification of high concentrations of mercury among coastal human residents, a quantitative comparison of the concentrations between species was not possible. Human exposure was assessed in hair and dolphin exposure in skin and blood. Second, although both dolphins and humans consumed fish from the same environment, the species of fish consumed differed. IRL dolphins consume a majority of their diet as striped mullet, silver perch and spot [39
] whereas human exposure was driven by those who reported consuming cobia, snapper, grouper and sea trout. Neither the human nor the dolphin data permitted a direct examination of methylmercury concentration, the most toxic form of the element. Since the participants were obtained through opportunistic rather than random sampling, the results cannot be generalized. Selection bias may well have occurred if the volunteer participants recruited through opportunistic sampling had higher concentrations of mercury in hair than non-participants. However, this sample of Florida residents contained substantial numbers of persons who obtained most or all of their seafood from commercial sources, permitting comparison of mercury exposure across frequency of seafood consumption and source of fish and shellfish. In addition, information bias resulting in under- or over- estimation of seafood consumption during the prior three months could have occurred. Since participants had no knowledge of their hair mercury concentrations at the time of interview, this form of bias would be expected to be non-differential with respect to their hair mercury concentration and lead to underestimation of risk.
In summary, these data provide insight into regional mercury exposure in Florida where access to and consumption of seafood is higher. In addition, mercury contamination in generally higher in Florida compared to all other states [16
]. Since our sample population is highly exposed to mercury, targeted education and local advisories should be designed to reduce regionally specific exposure pathways. Future local consumption advisories may include several of the species identified in this study, particularly for pregnant women. In contrast, there are well-recognized benefits of fish consumption for pregnant women and the general population. Positive effects on early cognitive development in children and a reduced risk of cardiovascular disease in adults have been attributed to the n-3 polyunsaturated fatty acids found to varying degrees in commonly consumed fish species [40
]. The challenge for public health is to find and recommend the balance between the positive and negative effects of fish and shellfish consumption.