Mercury is a big health hazard. I have recently posted on airborne mercury-containing nanoparticles. This article by Ed Yong is a good reminder that marine organisms we consume are still the main source of mercury.
Released by coal-burning power plants and other industries, mercury—a toxic metal—circulates in the atmosphere, enters the ocean, worms up the food web and, via the seafood we eat, ends up in our bodies. For decades mercury in seafood has been a health scourge, because it inflicts long-term harm on the brain and increases the risk of heart disease. It’s especially risky for developing fetuses, and mothers-to-be have long been warned away from mercury-rich tuna and swordfish.
Even though the mercury concentrations are falling rapidly due to falling coal use and due to regulations, the level of mercury in fish may actually increase!
Although there’s less mercury in the environment, our actions mean that fish like tuna are more likely to concentrate what’s already in their bodies. The carbon we pump into the atmosphere ends up affecting the amount of neurotoxin on our dinner plate.
Once mercury enters the ocean, microbes convert it to a compound called methylmercury, which then enters the food web. Each animal accumulates all the methylmercury in all of its prey, and all of its prey’s prey, and so on. So predatory fish, such as tuna, cod, and swordfish, accumulate the highest levels of the toxin, which they then bequeath to humans who eat them. In the U.S., 80 percent of methylmercury exposure comes from seafood, and 40 percent is from tuna alone.
In the 1970s, the gross overfishing of herring, the favored prey of Atlantic cod and spiny dogfish, forced these predators to switch to different targets. Cod moved on to other small fish, such as shad and sardines, which contain less mercury. Dogfish, however, moved to squid, which scavenge the bodies of animals further out on the food web, and so contain more mercury than expected for creatures of their size. As the herring recovered, both cod and dogfish returned to eating them. So since the ’70s, mercury levels have increased in cod, and decreased in dogfish.
Of course, the dose makes the poison. Are mercury levels in fish, current or future, relevant to human health? The answer, Sunderland says, is yes—and always yes. Epidemiological studies have shown that mercury exposures are linked to impaired brain development and cognitive abilities, especially when people are exposed in the womb. (Mercury can cross from a pregnant mother’s bloodstream into her fetus.) “It doesn’t look like there’s a threshold,” Sunderland says. “Everyone would like to see less methylmercury in their seafood. It’s never beneficial for human health.”