Fish Consumption Advice for Alaskans
Two large-scale epidemiologic studies, as well as numerous smaller ones, have examined the potential association between chronic low-level in utero exposures to mercury and subtle neurodevelopmental effects. One study took place in the Seychelles Islands off the coast of Africa and the other in the Faroe Islands in the North Atlantic between Scotland and Iceland. Because of the large sample sizes and the homogeneous nature of both study populations, the studies provide the best opportunity to characterize the magnitude and nature of the risks potentially associated with low-level methylmercury exposure through fish and/or marine
mammal consumption. Both studies have been reviewed and critiqued elsewhere.48,49 summarized briefly here.
The results are
The Seychelles Islands
In 1989, the University of Rochester, in collaboration with the Seychelles Island Government, initiated a large scale study (the Seychelles Child Development Study) of 779 mother-infant pairs, examining the developmental effects of low-level methylmercury exposure through frequent fish consumption.50-54 Seventy- five percent of the women indicated eating 10–14 fish meals per week.54 Mercury levels in 20 different species of fish (homogenized muscle) ranged from 0.001 ppm for reef fish to 2.04 ppm for Moro shark, and 4.4 ppm for dog tooth tuna.50 The overall average fish muscle tissue concentration was 0.3 ppm. Multiple maternal hair samples were collected during pregnancy for quantification of methylmercury exposures. Maternal hair mercury levels were as high as 27 ppm with a median of 6.6 ppm (compared to a maximum of 7.82 ppm and median of 0.53 ppm in 359 women of childbearing age in Alaska17). All but two women in the study had hair concentrations under 20 ppm, and 659 (80% of the cohort) had maternal hair concentrations 12 ppm. Maternal hair concentrations did not vary during pregnancy. Maternal hair mercury levels in each trimester correlated with levels representing the entire gestational period, indicating no seasonal differences or peak exposure periods.
Numerous neurodevelopmental tests and physical examinations were conducted on the children at 6.5, 19, 29, and 66 months of age. The neurologic evaluation included the Fagan Test, the Revised Denver Development Screening Test, the Bayley Scales of Infant Development, the General Cognitive Index, the Infant Behavior Record, Mental Developmental Index, McCarthy Scales of Children’s Abilities, Psychomotor Developmental Index, Preschool Language Scale, and numerous other perceptual, verbal, memory, behavior and motor tests.
No adverse health effects resulting from prenatal or postnatal exposure to methylmercury were noted in the 66-month evaluation, or in any of the earlier tests.52 In fact, greater prenatal and postnatal exposure to methylmercury correlated with better performance on some test scores, an outcome that may have resulted from beneficial effects of increased fish consumption. A new cohort has been established in the Seychelles to investigate the benefits of fish consumption versus the potential risks of methylmercury exposure. 55
During a subsequent follow-up of this cohort at age 9 years, tests previously reported to show an adverse association with prenatal exposure to methylmercury in the Faroe Islands were used.56 Investigators tested cognition (memory, attention, executive functions), learning, perceptual, motor, social and behavioral abilities. Of the 21 end-points evaluated, only two showed a significant association with prenatal methylmercury exposure. One association was adverse (the grooved pegboard, non-dominant hand) and the other association was beneficial (Conner’s Teacher Rating Scale, ADHD Index). As predicted, effects from other covariates known to affect child development were found. Consistent with the previous evaluations of this cohort, the investigators concluded that the findings did not support an association between prenatal exposure to methylmercury from consumption of large quantities of a wide variety of ocean fish and adverse neurodevelopmental consequences. 56