A study published this month found that mercury levels in smallmouth bass are about twice as high on average in fish in reservoirs along the Snake River compared with bass in free-flowing sections of the river.

The study included Brownlee, Oxbow and Hells Canyon reservoirs.

The research was conducted by the U.S. Geological Survey and Idaho Power Co. Results were published in Environmental Science & Technology.

Researchers tested 1,815 bass, of various sizes and ages, caught in various seasons in fall and spring 2013, 2015 and 2020 from 25 sections along a 530-mile reach of the Snake River between American Falls Reservoir in eastern Idaho and the northern end of Hells Canyon near Lewiston, Idaho.

Idaho Power employees caught the bass using electronic current to temporarily stun fish. The fish were caught in random sites. The fish were euthanized and tested in a lab to determine their mercury levels.

The river sections sampled included free-flowing reaches, reservoirs and tailraces, the latter defined as the section starting at the outflow of a dam and continuing for 15.5 miles or to the start of the next reservoir if that’s closer than 15.5 miles.

Researchers sampled smallmouth bass in three size classes — less than 5.9 inches, between 5.9 and 9.8 inches, and longer than 9.8 inches.

Researchers chose smallmouth bass because that species is distributed throughout the section of river studied, and because the species is popular among anglers, said Collin Eagles-Smith, a research ecologist for the USGS in Corvallis and one of the study’s authors.

Eagles-Smith said the study was an outgrowth of research the USGS and Idaho Power have been doing for many years, focusing on mercury levels in fish in Brownlee, Oxbow and Hells Canyon reservoirs.

One of the study’s goals, he said, was to see whether mercury levels were higher in bass in reservoirs, where it’s more likely that the river is stratified by temperature — meaning the warmer water near the surface doesn’t mix with colder water near the bottom.

“Impoundments, or dams, are one of the most common man-made changes to river systems,” said James Willacker, USGS ecologist and lead author of the study. “The reservoirs resulting from these dams along the Snake River can affect mercury cycling. We wanted to find out the degree to which fish mercury levels differed across reservoirs and their upstream and downstream environments, and how that translated into potential exposure and health risk to fish, wildlife, and humans.”

Whether the river is stratified based on temperature can contribute to the amount of methylmercury — a form of the toxic metal that can accumulate in fish tissue and then be absorbed by people who eat fish — that is produced, Eagles-Smith said.

Mercury can cause a variety of neurologic problems in humans.

Stratification and mercury

Here’s how the process can work:

When the surface water in a reservoir warms, algae are more likely to form, Eagles-Smith said.

That algae, which are fed in part by nutrients that enter the water from crop fertilizer, eventually sink and are fed on by bacteria. This process leaches oxygen from the water, creating ideal conditions for microbes to convert inorganic mercury, which is less readily absorbed by fish and people, into more easily absorbed methylmercury, Eagles-Smith.

There are several sources of inorganic mercury, he said, but typically the biggest contributor is airborne mercury, much of which results from burning coal. Airborne mercury enters rivers and reservoirs through rainfall and directly from the air, he said.

The researchers didn’t examine or estimate sources of inorganic mercury in the Snake River as part of this study, however.

The study results showed that mercury concentrations in bass is higher in reservoirs, where stratification is more likely since the water isn’t moving as quickly as in free-flowing reaches, Eagles-Smith said.

About twice as many bass in the largest size class caught in reservoirs or tailraces, compared with those from free-flowing sections, had mercury levels above the threshold the U.S. Environmental Protection Agency has set for determining whether people should limit how much fish they eat, according to the study.

Researchers also found that the highest mercury concentrations in bass were in reservoirs or tailraces that were stratified by temperature.

Northeast Oregon reservoirs

Among the 25 sites surveyed, mercury concentrations in the largest size class of bass — longer than 9.8 inches — were highest in Hells Canyon Reservoir, while the tailrace below Oxbow Dam had the second-highest. The Hells Canyon tailrace ranked sixth, Brownlee Reservoir was 11th, the Snake River below Hells Canyon Dam was 14th, and Oxbow Reservoir was 17th.

Anglers can only keep bass 12 inches or longer in Brownlee and Hells Canyon reservoirs. Oxbow Reservoir does not have a minimum length for bass, but anglers can’t keep bass between 12 and 16 inches long.

The lowest mercury concentrations were in the tailrace below Bliss Dam, which is near Twin Falls, in Swan Falls Reservoir south of Boise, and in free-flowing section of the Snake River upstream from Brownlee Reservoir.

Eagles-Smith said researchers will use data from the study to look for potential reasons why, for instance, the mercury concentrations in bass from Hells Canyon were higher than in neighboring Oxbow and Brownlee reservoirs.

“It’s too early to speculate on that, but we have a team working on that,” he said.

Multiple factors could affect the mercury levels in different reservoirs, he said, such as what the bass eat and how fast they grow.

Mercury levels can also differ between fish species in the same reservoir, Eagles-Smith said, although that, too, was not part of the recently published study.

Other species that are frequently caught, and eaten, from the Snake River and its reservoirs include crappie and catfish.

How to reduce mercury levels?

This question wasn’t part of the researchers’ task with the Snake River study, Eagles-Smith said.

He hopes, though, that the data they collected could contribute to future research into ways to potentially reduce mercury concentrations in bass and other fish.

The factors that contribute to oxygen-depletion in the water — a condition that contributes to the conversion of inorganic mercury to methylmercury — are an important part of the equation, Eagles-Smith said.

Reducing the amount of nutrients in the river that help algae form could be an important component, he said.

The study also notes that microbes converting inorganic mercury to methylmercury in stratified reservoirs isn’t the only source of methylmercury concentration in bass and other fish.

Sediment at the bottom of reservoirs is another source, according to the study.

Factors such as seasonal changes in reservoir levels might contribute to “elevated mercury concentrations in some reservoirs,” the study states.

Oregon fish consumption limit advisories

The Oregon Health Authority recommends people limit the amount of fish they eat from the Snake River and its reservoirs, including Brownlee, Hells Canyon and Oxbow, due to mercury concentrations. The guidelines apply to all fish, not just bass.

The agency recommends people in the “vulnerable population” — those under 6, women of childbearing age and people with thyroid or immune system problems — to eat no more than three meals of fish per month. The guideline for all others is to limit consumption to no more than eight meals per month.

A meal is defined as 1 ounce of uncooked fish per 20 pounds of body weight — a meal consists of 8 ounces for a 160-pound adult, or 4 ounces for an 80-pound child.

The OHA also recommends limiting the consumption of yellow perch from Phillips Reservoir, also due to mercury levels, to two meals per month for vulnerable people and five meals per month for others.

The guidelines for Owyhee Reservoir — for all fish species due to mercury — is one meal per month for vulnerable people and three meals per month for others.

A recently published study of mercury concentrations in smallmouth bass from the Snake River and its reservoirs in Idaho and Oregon measured mercury by milligrams per kilogram of weight (mg/kg).

The U.S. Environmental Protection Agency’s threshold for a mercury level that could pose a risk to people who eat fish is 0.30 mg/kg.

Average mercury levels in bass 9.8 inches or longer from river and reservoir sections in and near Baker County:

Snake River above Brownlee Reservoir

• 0.116 mg/kg (21st highest of 25 sites)

Brownlee Reservoir

• 0.277 mg/kg (11th highest)

Oxbow Reservoir

• 0.163 mg/kg (17th highest)

Oxbow tailrace (below dam)

• 0.480 mg/kg (2nd highest)

Hells Canyon Reservoir

• 0.555 (highest)

Hells Canyon tailrace

• 0.359 (3rd highest)

Snake River below Hells Canyon tailrace

• 0.230 (14th highest)