Since the 1970s, the amount of mercury found in fish in the Great Lakes has been on a steady decline.
Scientists have discovered that in Great Lakes species such as walleye and lake trout, mercury levels are rising. And they don't know why.
"We've been monitoring since the 1970s, and the (mercury contamination) trends overall have been declining--as have been the emissions of mercury into the atmosphere and deposition into the lakes," says Agnes Richards with Environment Canada. "We decided to look at recent trends, from 2000 to 2015. What we found is, at some specific sites, trends have reversed."
As far as scientists can tell, this shouldn't be happening at all.
Since the 1970s, Midwestern smokestack pollution has dropped. Technology to prevent pollution has been put in place. Even big polluters such as coal-fired power plants and factories have gone offline. But still, the contaminant levels in fish have been found to be on the rise.
At this time, scientists only have theories. One is that the warming trend of the Great Lakes could be a factor. Another is that the area's more frequent and intense storms has loosened buried sediments.
Finally, invasive species could be a factor by disrupting the diet of native fish. Scientists say that the population increase of goby and zebra mussels "rather neatly correlate" with the increasing mercury levels in Great Lakes sport fish.
At this time, scientists are monitoring the situation, since the rising levels could simply be a temporary fluke, rather than a trend.
Mercury levels in Great Lakes fish aren't exceeding limits set by the U.S. Environmental Protection Agency.