Catch and Release?

The historic headwaters of early research on mercury pollution and fish is right here on the Mississippi River, which was tested in 1969 and found to be deserving of its nickname, the Big Muddy.

“The first testing of mercury in fish found that there were high levels in the Mississippi, and people thought, ‘That makes sense—it’s polluted,’ ” says Daniel Engstrom, director of the St. Croix Watershed Research Center at the Science Museum of Minnesota. Far more startling at the time was the discovery that some of the major lakes in Voyageurs National Park—presumably as pristine as any waters in the world—were similarly sullied.

As scientists have since learned, most of the mercury that contaminates fish comes from human sources—coal combustion, mining, manufacturing, and the burning of products that contain mercury—and those emissions are carried by the wind and deposited on land and water. Micro-organisms in the water can convert the mercury into a more toxic form—methylmercury—which is, in turn, absorbed by other organisms, then eaten by small fish and then by larger predators, each link in the food chain absorbing potentially higher concentrations of the toxin in a process known as bioaccumulation. A powerful neurotoxin, methylmercury is especially harmful to brain development in children. Elevated mercury levels in wildlife have also been linked to motor skill impairment, hormonal changes, and reduced reproductive success. One study estimates that 8 percent of U.S. women of childbearing age have blood mercury levels in excess of what the Environmental Protection Agency deems safe.

By 1975, state health officials had begun issuing the now familiar fish consumption advisories that go something like this: Choose pan fish such as sunnies, crappies, and perch, which are less likely to have built up high levels of mercury. Go for the smaller game fish and trim away skin and belly fat, where PCBs (synthetic oils that were banned in 1976, but decompose slowly) can build up. Because there’s no way to clean or cook a fish to eliminate the mercury content, limit meals of larger fish such as walleye, bass, and lake trout to one meal a week for men and once a month for children or women of childbearing age. Any fish large and mature enough to win a contest is probably better mounted on a plaque than served on a plate.

Anglers interested in the fish stock of a particular lake can also turn to the Lake Finder feature created by the MDH and the Department of Natural Resources, analyzing more than 1,000 popular lakes and rivers. There you may learn that women of childbearing age and children should limit their meals of smallmouth bass from the Lake of the Woods to once a month, though they can eat unlimited amounts of sunfish caught from the high-traffic waters of Lake Minnetonka. These readings can seem counterintuitive, but the fact is, the complex balance of wetlands, bacteria, and watershed makes some of the cleanest lakes in the state especially efficient at converting mercury into methylmercury.

While mercury remains the top health concern, the state monitors other pollutants as well. In April, the MDH issued a fish consumption advisory for Lake Calhoun and parts of the Mississippi south of Minneapolis after the chemical perfluorooctane sulfonate—a chemical once used in 3M’s Scotchgard and other products—was found in fish taken from those waters. Now anglers are advised to limit meals of bluegill caught in Lake Calhoun and its connected chain of lakes—Brownie, Cedar, Lake of the Isles, and Harriet—to just once a month.

Minnesota is a national leader in fish-monitoring programs, in part because we have so many lakes and rivers. In 1994, the MDH became the nation’s first health department to integrate consumption advisories for fish caught in the state with warnings about commercial fish caught elsewhere. “MDH felt that all sources of methylmercury exposure should be considered when making choices about which fish to eat and how often,” McCann explains. “We also wanted to inform women who were eating Minnesota sport fish that it’s not necessarily safer to substitute commercially available fish.”