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A lake trout is removed from a gill net on Quartz Lake. Most of the juveniles are between 8 and 12 inches long.

Fish are good for you: Eat more.

Fish are full of toxins: Eat less.

Both statements can be true, but the evidence for either claim rarely gets paired with the other. That can make it tough to decide what to do with that creel of trout you brought home from a good day on the lake.

“Many of us think of freshwater fish as just as good a source of healthy fatty acids,” said Dartmouth College research professor Celia Chen, a project leader for the school’s Toxic Metals Superfund Research Program. “In other places, it’s a major environmental hazard. There has to be more discussion around risk-benefit, instead of just risk.”

Chen led a team that looked at both the healthy components and contaminants found in 63 fish species over the past two decades.

The review made two important conclusions: Researchers rarely test for both good and bad factors in nutritional analyses, and those good and bad components can vary widely within a single species of fish, depending on harvest times and locations.

In Montana, the state Department of Fish, Wildlife and Parks has posted notices of what game fish are better or worse for regular eating. The notice includes location-based charts advising the species and sizes of recommended edible fish by river system or lake.

For example, the Clark Fork River below its confluence with the Blackfoot River east of Missoula has warnings against eating any size northern pike and suggests limiting meals of 10- to 18-inch rainbow trout to four times a month.

It also has warnings about the presence of toxic furans and PCB chemicals around the closed pulp mill near Frenchtown. But many of those fish aren’t eaten anyway.

“There’s the trout community that pushes catch-and-release a lot more,” said FWP water pollution biologist Trevor Selch. “We manage our rivers and streams based on natural reproduction, where we let the ecology take care of itself. We only stock lakes and reservoirs, and I think we’re the only state in the country that does that (limited stocking).”


Properly prepared fish provide meals with high protein, low saturated fats and beneficial compounds like Omega-3 fatty acids that can prevent heart disease. But they can also deliver contaminants such as PCBs, methyl mercury, selenium, cadmium and other toxins that are bad for human health.

“That’s a recurring topic, trying to make people aware of toxicity, but not deter them,” Selch said. “Sometimes it’s their best source of healthy protein.”

Mercury can occur naturally in the environment or as a result of mining activity, industrial waste, or household refuse such as batteries and thermometers. It can cause neurological damage in children or developing fetuses in pregnant women.

PCBs come from industrial waste, and can cause developmental disabilities, immune system damage and cancer. Heavy metals such as arsenic, cadmium and selenium often result from coal-burning power plants or mine waste, and cause a variety of health problems.

In the Dartmouth paper, Chen reported that in the next decade, total global production of aquaculture and wild fisheries is expected to exceed that of the poultry, pork or beef industries.

Most of Chen’s research looked at saltwater fish species. There, she found factors like ocean warming, acidification and changes in runoff from human activity such as agriculture or waste disposal can greatly affect the nutritional value of fish.

She also reported that freshwater fish can vary in their environmental exposure. For example, one project she reviewed found high levels of mercury contamination in mayflies, but the toxin wasn’t working its way up the food chain to the fish.

In Montana, health guidelines recommend avoiding eating lake trout over 28 inches long, walleye over 22 inches long, northern pike over 28 inches long, and bass over 15 inches. Those predatory fish have reached a size where their bioaccumulation of toxins could cause adverse health effects.

Smaller specimens of those four species should be kept to one meal a month, while catches between 15 and 20 inches of lake trout, northern pike, brown trout and rainbow trout are limited to one meal a week.

Lake trout smaller than 15 inches, bass smaller than 11 inches, brown trout smaller than 15 inches, yellow perch smaller than 10 inches, whitefish smaller than 18 inches, and most cutthroat trout, brook trout, kokanee salmon, sunfish and arctic grayling are OK to consume twice a week.

“Anything that’s longer-lived or eats other fish has typically your highest level of contamination,” Selch said. 

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Natural Resources & Environment Reporter

Natural Resources Reporter for The Missoulian.