But over the past several years, the decline has reversed. Scientists are finding mercury levels rising in large Great Lakes fish such as walleye and lake trout. Curiously, it’s occurring in fish in some locations but not others. Researchers are trying to figure out why. The mercury levels are not surpassing U.S. Environmental Protection Agency thresholds, but researchers want to determine whether what they are seeing is temporary or a trend that will worsen. The answer has large ramifications for the Great Lakes’ sports-fishing industry. Mercury is a heavy, silvery metal, unusual in that it’s liquid at room temperature. It’s naturally occurring, but is rare to find uncombined with other elements.