As a top predator on the food chain, sharks and other large predatory fishes are known to accumulate toxins acquired from the fish and other prey they eat throughout the course of their lives. Several published papers indicating that shark meat contains high levels of methyl mercury, which is a potent neurotoxin that is often derived from industrial processes, notably cement manufacturing.
Results from a recent study conducted by MarAlliance team members indicated that over 80% of sampled sharks at Lighthouse Reef Atoll in Belize were above the US Environmental Protection Agency (EPA) advisory levels of 0.3 parts per million (ppm) and over 37% were above the Federal Drug Administration (FDA) and World Health Organisation (WHO) advisory levels of 1.0 ppm. Previous research on methyl mercury levels in Goliath grouper conducted in Belize by MarAlliance suggests that mercury levels are high in animals larger than 55 cm TL, with many exceeding the US governmental advisory criteria for human health (Evers et al. 2009).
Although alarming, these results are not wholly unsurprising. Sharks and groupers are apex predators that are known to biomagnify toxins, including heavy metals such as mercury, due to the many food chain linkages existing in the marine environment. In humans, absorption of large pulses of mercury and continued ingestion above the advisory levels impacts cognitive abilities, memory and depresses serotonin levels.
US EPA and FDA advisories suggest that pregnant women should avoid eating fish such as shark and king mackerel and not eat more than two meals of fish or shellfish of lower mercury content a week (no more than 12 ounces or two average meals (see: What You Need to Know About Mercury in Fish and Shellfish). Because of the potential for developmental problems, these advisories are considered too high for pregnant women and young children, with suggestions that 0.12 ppm would be the acceptable threshold for these groups.