Last month, the U.S. Senate voted 98-1 to approve a resolution saying that “climate change is real and not a hoax.” Just a year or two ago, it would have been laughable to hear both Democrats and Republicans agreed to this, but a lot has happened in that time. Parts of Antarctica are melting three times faster than they should be and Iceland, losing all its ice, is starting to see its land rise. The effects of climate change are widespread, affecting life on the planet as we know it. And one of those effects, a new study finds, is a rise in mercury levels among open-ocean fish like tuna.

Average global sea surface temperatures have been higher over the past 30 years than at any other time since the Environmental Protection Agency first started tracking those temperatures in 1880. This rise in surface temperatures has changed the way marine animals live; tropical fish are moving north into waters that used to be cooler, whales are losing food sources. The warmer waters are also changing fish’s appetite and metabolism. A 2013 study found killifish, little forager fish that don’t tend to eat much, eat a lot more in warmer water. That means the little fish are consuming more methylmercury — the organic toxic form of the element — and passing it on to larger predatory fish like tuna and swordfish.

Researchers from the University of Michigan found concentrations of mercury in Hawaiian yellowfin tuna increased at least 3.8 percent each year from 1998 to 2008. That’s bad news for the millions of people who eat the fish, which is commonly marketed as ahi, and used in raw dishes like sashimi or grilled to sear the edges. Exposure to high levels of mercury can affect the brain, heart, kidneys, lungs, and immune system, leading to blindness, deafness, impaired cognitive function, muscle atrophy and twitching, kidney malfunction, and respiratory failure. Damage caused by the element is irreversible.

For the study, researchers looked at data from three studies that looked at yellowin tuna from 1971, 1998, and 2008 — all three studies analyzed mercury levels in the fish’s muscle tissue. Then, using a computer model that controls for body size, they analyzed 229 fish between 48 and 167 pounds, and calculated mercury concentrations. They found that mercury concentrations didn’t change between 1971 and 1998. However, fish from 2008 had concentrations much higher than either of the other years.

“Mercury levels are increasing globally in ocean water, and our study is the first to show a consequent increase in mercury in an open-water fish,” said lead author Paul Drevnick, an assistant research scientist at the university’s School of Natural Resources and Environment, in a press release. “More stringent policies are needed to reduce releases of mercury into the atmosphere. If current deposition rates are maintained, North Pacific waters will double in mercury by 2050.”

Mercury is a common air and water pollutant due to its use in manufacturing products like fluorescent lights, batteries, and latex paint. It becomes methylmercury when it interacts with bacteria in water, and also ends up in the ocean through natural events like volcanic eruptions. Exposure to the methylmercury is especially dangerous because it doesn’t quickly break down in the body, which is why experts advise eating fish — particularly the bigger ones — in moderation, and for pregnant or nursing women to skip it altogether.

Source: Drevnick P, Lamborg C, Horgan M. Environmental Toxicology and Chemistry. 2015.

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