MISSOULA — People have messed up the food web of Flathead Lake in so many ways, from introduced species to increasing levels of mercury. Now, with the help of a federal grant, the Flathead Lake Biological Station will study how all those issues interact so it can model how much mercury people might be eating in their fish.
Last week, the U.S. Environmental Protection Agency gave the University of Montana Flathead Lake Biological Station a grant of almost $129,000 to work with the Confederated Salish and Kootenai Tribes to study four Flathead Lake species to provide a baseline measurement of methylmercury in the aquatic food web.
Then, once they have a better idea of mercury levels, they can determine how much trout certain CSKT groups are eating and whether it’s safe.
Research economist Nanette Nelson said the CSKT effort to reduce the population of lake trout in Flathead Lake sparked the idea to do the study.
“These big hogs of lake trout, the ones that come out that are about 40 pounds, they’re just chock-full of mercury,” Nelson said. “Our question is, so you start removing those big guys, what happens to the rest of the food web?”
Non-native lake trout were introduced into Flathead about a century ago but weren’t as much of a problem until the 1980’s. That’s when mysis shrimp – planted in five upstream lakes from 1968 to 1976 to improve kokanee salmon populations – drifted into Flathead Lake where they ravaged the zooplankton population and fed ravenous lake trout. The lake trout then out-competed the kokanee in Flathead Lake, completing the destruction of the native food web.
In an effort to turn that around, the CSKT in 2014 published a plan to try to turn that around by hammering the lake trout population. Using fishing derbies and gill netting, the tribes set a goal of removing 143,000 lake trout a year to eventually reduce the population – estimated at around 1.5 million – by 75%.
But sponsoring fishing derbies and running gill nets around the lake every fall isn’t cheap, and the tribes are probably going to have to do both for decades if not forever. To offset the cost, in 2017, the CSKT approved the creation of the nonprofit Native Fish Keepers, Inc., which processes, freezes and sells lake trout fillets.
“This is so expensive that the revenue we generate through Native Fish Keepers will never completely offset the costs, but we are really encouraged so far by the popularity of the product,” CSKT Fisheries Biologist Barry Hansen told the Flathead Beacon in 2018. “The demand for it is high, so the potential is great.”
The high demand needs to be tempered with caution, because eating too much lake trout can lead to mercury poisoning.
Mercury enters lakes and rivers from the air. A couple centuries ago, when the only sources were volcanoes, certain rock formations, geothermal vents and wildfires, mercury wasn’t really a problem. But during the last 150 years, human activities such as mining, coal combustion, waste incineration and industrial processes, have more than doubled the amount of mercury in the atmosphere.
Mercury alone is not a health concern. But in the oxygen-deprived depths of a lake, bacteria can turn mercury into methylmercury, a toxin that attacks the nervous system.
More than 95% of all mercury in fish is methylmercury, which becomes more concentrated the higher up the food chain you look. For example, if you ate an 8-inch whitefish, it would have lived only about a couple years and eaten only a limited number of insect larvae or snails that contain small amounts of methylmercury.
But if you eat a 30-inch lake trout, it sits higher up the food chain and will have eaten many of those little whitefish and other trout. So it would have accumulated a lot more methylmercury.
After sampling the mercury levels in lake trout, the CSKT and Montana Fish, Wildlife & Parks issued a lake trout consumption limit in 2014 for everyone but especially for children and women of childbearing age. No one should eat lake trout longer than 30 inches, and women and children should limit themselves to one serving a month if the trout is 22 to 26 inches long.
This fall, the Flathead Lake Biological Station study will take advantage of the fall gill-netting to measure the mercury concentration in lake trout tissue.
Then, scientists will extend that farther down the food chain by also measuring mercury concentrations in whitefish, mysis shrimp and Daphnia, a group of zooplankton. The biological station already runs a decades-old monitoring program that collects shrimp and zooplankton. They’ll just add mercury to their regular analysis.
They’ll process all the samples and probably have some preliminary data within nine months, Nelson said. Once a model is developed to create a graph, the result will probably resemble a straight-line or slightly bowed curve with methylmercury concentration increasing as you move higher up the food chain.
That outcome seems obvious, although the analysis might yield a surprise or two.
But the real goal is to repeat the sampling after the lake trout population has dropped in about a decade or so and see if the curve changes. The result could provide a glimpse back in time to what the food web looked like in the 1980s when the lake trout were first taking off.
This information could then be used to predict what might happen in other lakes if invasive species were introduced.
“Invasive species have a tendency to really mess up food webs or alter them from what we guess was happening before we showed up,” Nelsons said. “Plus, mercury is this really complex element when it gets out in the environment, and it has so many things that can play a role in how and where it bio-accumulates. So that’s a big question: how do the changes in food web dynamics then alter these bio-accumulations of mercury within the organisms?”
The Flathead Lake curve might change because a smaller lake trout population means the lake trout that remain don’t have to compete as much and they might grow faster. A lake trout of a certain size might be younger in the future, so it would have eaten fewer fish and would contain less methylmercury. Also the whitefish population may grow.
Both changes might cause the upper end of the curve to shift a bit, and they might require a change in the fish consumption advisory.
That’s where Nelson, as an economist, comes in.
Native Fish Keepers also donates lake trout fillets to CSKT food pantries, which service up to 400 low-income tribal families. One advantage of the food pantries is they dole out food only once a month, so consumption guidelines aren’t exceeded because no one is getting more than one serving of lake trout a month, Nelson said.
The fillets are a good source of protein and beneficial fish oils, but combining them with other fish such as canned tuna, fish sticks or other trout, could lead to mercury poisoning.
That’s why Nelson will conduct as many interviews as possible with these families to find out how much fish of all kinds they eat and whether they’re aware of fish consumption advisories.
“That information will help the tribes determine if they need to do more in terms of getting the word out,” Nelson said. “It’s a difficult message to get across to people. On the one hand, there are these great benefits from eating fish, but it also can be bad because of the mercury.”
The UM award was one of the first 14 grants totaling about $2 million from the Columbia River Basin Restoration Funding Assistance Program, established by Congress in 2016 in part to reduce toxins affecting the health of the waters throughout the basin. A similar project in Oregon and Washington will look at toxins, including mercury, in the tissue of fish and invertebrates along the 600 miles of the Columbia River.