Most everybody has heard by now that we need to limit the intake of some fish, as it accumulates a lot of toxic mercury. Anything who consumes fish in the food chain is consuming high levels of the mercury the fish has accumulated as well, including humans.
Seafood has mercury in it because of accumulation in the food chain. Plankton is at the bottom, and they accumulate mercury. Plankton is eaten by small fish, and the mercury moves into their bodies. Small fish are then eaten by large fish which absorb the greatest amount of mercury.
Fish that are predatory (eat other fish) are larger and higher up in the food chain, so they generally contain more mercury. Fish that consume prey in certain ways can have a much higher concentration of mercury than other species.
Some varieties of fish with piscivorous behavior (fish-eating behavior) are generally found to contain over 0.5 parts per million total mercury. In fact, larger, predatory fish can contain concentrations of mercury that are as much as 10 times higher than fish that are consumed by humans. Large predatory fish, like tuna, for instance, contain methylmercury about 10 million times higher than that found in the surrounding water. In any given body of water, the highest concentrations of methylmercury are typically found in the largest fish, which consume other fish.
Methylmercury in the Seafood Supply
Methylmercury is the most abundant form of mercury in fish and is present in certain types of fish in concentrations that can harm human health. Fish and shellfish accumulate mercury in their bodies, usually as methylmercury, a highly toxic organic mercury compound.
Microorganisms in aquatic bodies can convert Mercury to methylmercury, which accumulates in the fish and shellfish. Mercury concentrations in the air are not of much concern, but when the mercury gets into the water, the microorganisms convert it to the highly toxic form — methylmercury — which builds up in fish.
Traces of mercury, mostly in the form of methylmercury, are found in almost every species of fish, though some fish have higher levels than others. The type of mercury that accumulates at toxic levels in fish is called monomethylmercury, or just methylmercury because it has a methyl group, CH 3, attached to the atoms of the mercury.
The most dangerous form of mercury is monomethylmercury, which living things like fish and humans cannot get rid of easily, so it builds up in their tissues to high, toxic levels. Living things like fish and humans Monomethylmercury. If/when humans eat a few long-lived fish, the mercury may build up within them, and if their mercury levels become too high, they may get mercury poisoning.
Mercury Accumulates in Older Fish
Long-lived fish, particularly predatory and/or larger ones, may have a lot of mercury that builds up over time inside the long-lived fish. For biomagnification, longer-lived fish and higher-ranking predators such as swordfish and sharks typically have the highest levels of mercury. Nearly all fish have trace amounts of mercury, but long-lived predators – such as tuna, swordfish, and sharks – typically have higher levels. Some fish, though, contain higher levels of mercury — enough to harm developing fetuses or infants.
However, some fish and shellfish contain high levels of mercury, which can damage the developing nervous system of a developing fetus or infant. Those fish that in general can contain higher levels of mercury include bluefish, sharks, swordfish, wild sturgeon, large-eye tuna, bluefin tuna, redfish, and several others. For example, those fish that in general have lower levels of mercury include shrimp, scallops, sardines, wild/Alaska salmon, tilapia, and many others.
Five of the most frequently consumed mercury-free fish are shrimp, canned light tuna, salmon, pollock, and catfish. Fish species that are longer-lived and higher up in the food chain, such as redfish, tuna, shark, swordfish, king mackerel, and tilefish (Gulf of Mexico), have higher concentrations of mercury than others.
Sharks Have The Highest Mercury Levels
Australian studies suggest mercury levels in certain fish, especially shark, maybe even higher than those found in the region studied in this research. The findings indicate that higher levels of mercury found in tuna and other ocean fish could be coming from a natural source rather than from contamination. Studies have been conducted on brain development in children whose mothers have eaten a substantial amount of fish at elevated levels of mercury in pregnancy, in New Zealand, the Faroe Islands, and Seychelles.
A risk-benefit study that evaluated risks from mercury intake versus benefits derived from salmon in Alaska found that benefits exceeded risks in both cardiovascular health and infant neurological development from salmon consumption, and that data on mercury methylation in non-oily fish must be high-quality before the relative risks can reliably be identified. These researchers recommended that fish known to contain mercury should be reduced in diets, especially for pregnant women.
For pregnant or nursing women, and young children, mercury risks are substantial enough that it is recommended that mercury-rich fish be completely removed from the diets of pregnant or nursing women.
Fish Still Has Nutritional Value
The nutritional benefits of fish can outweigh the risks, provided that high-mercury fish are limited. You can also get exposed by eating fish and shellfish, as those animals soak up lower levels of mercury from water contamination. The FDA and the Environmental Protection Agency (EPA) recommend that women who are likely to become pregnant, women who are pregnant, breastfeeding mothers, and young children avoid certain types of fish and to eat fish and shellfish that are low in mercury.
A 2014 survey by the United States Geological Survey on mercury levels in the United States water supply found methylmercury concentrations in fish were generally highest in wetland areas, including streams on the coastal plain of the southeast. Methylmercury levels in fish were also highest in the Western U.S., but only in streams mined for mercury or gold.
Another study found that a third of the fish caught off New Jersey’s coast had mercury levels above 0.5 parts per million (ppm)–a level that can lead to health problems in people who eat that fish regularly (3). The new research also needs to be extended to other predatory ocean-going fish, such as swordfish and sharks, according to Morel, which might mean that anything that passes mercury up from surface waters into the tuna is likely doing the same thing for other predatory ocean-going fish.
