Elevated mercury levels found in Lake McDonald bull trout

The U.S. Geological Survey recently reported finding elevated mercury levels in bull trout in Glacier National Park’s Lake McDonald that could be harmful to the fish and predators that eat them.

The study looked at mercury levels in fish across 21 national parks in 10 western states, including Alaska.

The mercury levels in the Lake McDonald trout, which were sampled back in 2008, “approached levels of concern for tissue-based fish toxicity.”

“Because we only sampled fish from a single lake in Glacier, it is unclear if this is a widespread concern for toxicity to bull trout or their predators, but our data suggest that a follow-up study is warranted,” study author Colin Eagles-Smith reported.

Of 15 bull trout sampled, mercury levels ranged from 167 nanograms per gram of fish with an average of more than 232 nanograms. One sampled fish had nearly 300 nanograms of mercury per gram, which is just below the recommended threshold for safe human consumption. A nanogram is one billionth of a gram.

Few humans, however, eat bull trout from Glacier Park — it’s illegal to keep and eat bull trout in the Park because they’re listed as threatened under the Endangered Species Act. But the mercury could be harmful to the bull trout themselves or animals like otters or birds that eat the fish.

While Lake McDonald fish had elevated levels of mercury, high elevation lakes in the Park had very low levels of mercury. In the previous study, Old Man and Snyder lakes were sampled in the mid-2000s by scientists from the Environmental Protection Agency. That study found fish had low levels of mercury but high levels of some pesticides, likely airborne from agricultural fields outside the Park.

The difference in the lakes’ structures and environments might be why there’s mercury in Lake McDonald and not higher elevation lakes, Eagles-Smith said. In order for fish to absorb the mercury into their bodies, the mercury needs to be methylized, where inorganic mercury is converted by microbes into methyl mercury. That process requires an aquatic ecosystem with low oxygen, organic matter, sulfates and iron — an environment found in Lake McDonald’s deep waters but not in shallower high mountain lakes, he said.

Eagles-Smith said it was common to find different mercury levels in lakes in the national parks depending on their individual environments. The study found low levels of mercury in most national parks, but it also suggested further study in Glacier Park.

Glacier did further studies in 2010, and found mercury in fish, but not above levels that were safe for human consumption.

Mercury is harmful to humans and wildlife and is among the most widespread contaminants in the world. Distributed at a global scale, mercury originates from natural sources, such as volcanic eruptions, and from human sources, such as burning fossil fuels in power plants.

Mercury distribution at local or regional levels can result from current or historic mining activities. Human activities have increased levels of atmospheric mercury at least three-fold during the past 150 years.