Climate change “is not just about what the weather is like in 10 years,” Schartup says. “It’s also about what’s on your plate in the next five.” 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. Read: Wait, so how much of the ocean is actually fished? This simple pattern hides a more complex one. Researchers have noted that trends in mercury levels can vary considerably between different species of fish, even those that live in similar environments. “And when people looked at trends, some would go up, some would go down, and some would be flat,” Schartup says. “Why, if they’re all experiencing the same declining mercury levels in seawater?” That’s especially confusing for regulators, who reasonably expect to see their emissions-curbing work lead to consistent benefits. To find out, Schartup and her colleagues collated three decades of data on fish stocks and mercury levels from the Gulf of Maine. They plugged that info into a model that simulated the region’s food webs, in which virtual fish grow up in virtual seas, eating virtual plankton, and accumulating virtual mercury. And by tweaking the model to account for changing environments over the past half century, they showed how human activities radically shaped the amount of mercury that gets into different fish. 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. “Everyone who’s looking at those different trends in fish: You’re not all crazy,” Schartup says. Read: A big, weird, pancake-shaped fish keeps surprising beachgoers Temperature matters, too. The water in the Gulf of Maine has warmed considerably since the 1960s, and more so than most other parts of the world’s oceans. Because most fish are cold-blooded, their physiology is yoked to the warmth of their surroundings. As oceans get hotter, they become faster and more active, they eat more prey, and they consume more mercury. (Even tuna, which can partly control their body temperatures, experience this effect, because everything else they eat has already built up more of the toxin.)
Levels of mercury in fish could continue to rise, despite concentrations of the element falling by 30% in the Northern Hemisphere between 1995 and 2010, due to climate change and overfishing. Both factors could force fish to consume more heavily contaminated prey, accumulating the mercury that it contains. This is passed on to humans through the food chain, and is linked to health threats including heart and brain disease. Of methylmercury exposure in the US, 80% comes from seafood, with 40% from tuna alone. Rising ocean temperatures also lead fish to eat more, thus increasing their mercury intake.