Understanding Mercury Concentrations in Mountain Lake Fish

Doubtful Lake in North Cascades National Park.

Ariana Chiapella

Mountain lakes may seem pristine, but they are subjected to multiple types of anthropogenic (i.e., man-made) stressors. Since the industrial revolution, toxins from industrial activities have traveled through the atmosphere and been deposited onto the mountain landscape, where lakes act as collection basins. These lakes were historically fishless, but starting at the turn of the 20th century, fish stocking became a regular practice in an effort to attract visitors and garner support for land conservation. Many mountain lakes now have reproducing fish populations, and studies show that these new residents lead to declines in lake invertebrates and amphibians. Mountain lake fish also accumulate toxins like mercury that are deposited from the atmosphere. These stressors have created a challenge for park managers, who want to protect the health of mountain lake ecosystems, the health of visitors, and the cultural value of fishing.

Approach

This study sought to determine how much mercury is in mountain lake fish, and to understand which variables contribute to high mercury in fish. The goal was to identify variables like water quality, elevation, tree cover, precipitation, lake shape, fish diet, and fish species that could be easy-to-measure indicators of high mercury in fish. To do this, 20 lakes were sampled across North Cascades, Mount Rainier, and Olympic National Parks in 2014 and 2015. Mercury was measured in fish, zooplankton, and benthic macroinvertebrates from each lake. Each part of the food web was also analyzed for stable isotopes, a tool used by ecologists to estimate what consumers – like fish – are eating based on their isotope “signature”. The variables collected were used to build models to determine which factors were most important in predicting fish mercury concentrations.

Microscope photo of a copepod zooplankton from a North Cascades mountain lake.

Ariana Chiapella

Results

Luckily, for park visitors who like to eat fish from mountain lakes, mercury concentrations in fish are below the EPA’s limit for what is considered safe for recreational anglers to eat. However, most fish exceed the EPA’s limit for subsistence fishing, which means many mountain lake fish may not be as safe to eat for those who rely on fish for a substantial portion of their diet. Fish from North Cascades National Park had significantly lower mercury than fish from Mount Rainier and Olympic National Parks, and rainbow trout had significantly lower mercury than cutthroat or Eastern brook trout. However, models suggest there are other factors at play. For example, lakes with a lot of tree cover tend to have fish with higher mercury than lakes with more scrub vegetation than trees. And surprisingly, lakes with higher annual precipitation have fish with lower average mercury than lakes with lower annual precipitation Fish size is also important: the bigger the fish, the higher its mercury content.

Mercury (Hg) concentrations in fish compared across Mount Rainier (mora), North Cascades (noca), and Olympic (olym) National Parks (A), and across fish species (B).

The solid red line indicates the EPA’s threshold for safe recreational consumption (300 ng/g), while the red dashed line indicates the threshold for safe subsistence consumption (49 ng/g).

Implications

While preliminary, these results can be used to inform fisheries management efforts, and can be used in outreach efforts. Easy-to-identify factors like tree cover are useful for fisheries management decision-making, as park managers aim to balance ecological health, visitor health, and visitor values. A complimentary study looks at the public perceptions of fisheries management in mountain lakes in national parks, as well as the fish consumption habits of park visitors. Together, these studies provide park management with new information that can be used to determine if fish should be removed from, or left to reside in mountain lakes.

Ariana sampling a lake in North Cascades National Park.

Researcher

Ariana Chiapella is a PhD candidate at Portland State University. Her dissertation investigates the factors that influence mercury in mountain lake fish, how mercury moves through food webs, and the social and management implications of contaminant accumulation in fish-stocked mountain lakes. Her research is inspired by her love of the mountains. Her interest in aquatic ecotoxicology began as an undergraduate, where she studied the accumulation of PCBs in crayfish from rivers in the Berkshire Mountains in Massachusetts.

Summary by Ariana Chiapella, December 2018. Published by the North Coast and Cascades Research Learning Center.