Based on a request to the Minnesota Department of Health, or DOH, we recently obtained an enormous database showing the results of mercury testing on almost 17,000 individual fish samples taken from lakes in St. Louis and Lake counties from 1971 through last year.
The data is eye-opening in several ways, but what was perhaps most noteworthy was the scale of the state’s testing program over the years. The state is clearly spending millions of dollars annually to monitor the health risks associated with eating fish. That should provide Minnesotans with some level of confidence.
The good news is that the fish in the vast majority of the lakes in our region are low in mercury and safe to eat. In fact, health officials repeatedly make the point that eating fish is good for you, since they are generally high in protein and healthy fats.
But the fish in a relative handful of our lakes are shockingly high in mercury, and it’s not entirely clear why that is. While some have pointed to sulfates as a factor, the results suggest other factors are involved as well. Over the next few months, I plan to further investigate these connections. As of today, I’m not ready to report any conclusions since I still need more information. One thing I can guarantee is that my reporting will be guided by the science.
One thing that I can report today with confidence is that anyone concerned about safely eating fish should pay close to attention to size. Using the Claude AI model, I was able to incorporate the MDH data to generate scatter point graphs that show both the length of fish and the mercury concentration in the flesh of each sample. I produced graphs for samples of walleye and separate graphs for northern pike.
The correlation between fish length and mercury concentration is striking. While there is always a little noise in test results, the correlation is very strong and it is even stronger in other lakes in the region, as future graphs will show.
Most of us who fish have undoubtedly heard about this connection before, but to see the data put on a graph really dramatizes the point. I added a regression line that shows the trend of each data set and for every lake I graphed the results were the same. I’ll have more information about specific lakes where fish tested very high for mercury in the coming weeks. Some of those lakes may come as a surprise to many.
One thing I can say for sure based on the data is that the region’s most popular walleye lake, Lake Vermilion, tests quite well as the graph included above attests. The overwhelming majority of fish sampled from Vermilion tested in the safe zone (shown in the green shaded portion of the graph), according to the Environmental Protection Agency. That’s anything below 0.5 parts per million, or ppm. Only a handful of walleye sampled from Vermilion tested in the caution zone and they were all 19 inches in length or above. As a general rule, you can assume if it’s under the protected slot, it’s highly likely to be low in mercury. If it’s over the slot, it’s a trophy, not something to put in the frying pan. Best to measure it, take a quick photo and put it back in the lake to help keep the population strong.
That is good advice for most lakes, but not all of them. There are some lakes in our region, which tend to be clustered in the northwestern part of the BWCAW and parts of Voyageurs National Park, that are so high in mercury that you probably should avoid eating fish from these lakes entirely. In these lakes, the mean mercury content of the fish well exceeds one ppm, which is the limit for the sale of fish commercially in the U.S.
Based on the guidance put out by the DOH, children under 15 and pregnant women or those who may become pregnant should avoid eating any fish from these lakes. And for the rest of us, the guidelines recommend no more than one meal per month from these lakes.
In most cases, the DOH doesn’t issue lake-specific guidelines, since the fish from most lakes don’t pose much risk to consumers. In those cases, the DOH recommends as many as four meals a week of fish like stream trout, crappies, sunfish, or perch and one meal per week with walleye, northerns, or bass.
It pays to pay attention to these guidelines because humans have known for years that mercury causes neurological impacts on humans, which can worsen over time. The connection to the consumption of fish gained international attention in the wake of the mass poisoning of Japanese citizens who ate fish caught near a chemical plant that dumped mercury into the ocean near Minamata.
While the concentrations of mercury in Minnesota lakes are extremely low by comparison, they concentrate or “bioaccumulate” in the larger predator fish at the top of the food chain. The more of these large predator fish, like walleye and northerns, we eat, the more the mercury accumulates in our bodies and that can affect us over time. Young children and infants are especially vulnerable.
The mercury found in our area lakes can come from a few different sources. It can be naturally present within rocks or glacial till in a lake’s watershed, which likely explains some of the worst cases in the area. It also falls from the sky as fallout from the burning of coal. While we had made some progress on that front in recent years, the Trump administration has now relaxed restrictions on mercury emissions, so this source could worsen if that policy actually takes effect.
Elemental mercury, by itself, is far less toxic than methyl mercury, which was the cause of the tragedy at Minamata. But elemental mercury can be converted or “methylated” to methyl mercury by a type of bacteria, known as sulfur-reducing bacteria, found in lake sediments. These bacteria use sulfates as part of that process, which is why sulfate levels matter in our lakes. More on that later.