Monitoring Mercury's Risk to Wildlife in California Lakes

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USGS scientists sampled grebes in California lakes to compare mercury levels in these predators versus the small fish they prey on—developing a new tool for water quality monitoring.

Estimating Mercury Exposure to Piscivorous Birds and Sport Fish in California Lakes Using Prey Fish Monitoring: A Tool for Managers

Many bodies of water in California are listed under the Clean Water Act as impaired by mercury contamination. California's Regional Water Quality Control Boards have been developing water quality objectives and standards for mercury to ensure that the beneficial uses of California's waters—including wildlife habitat—are supported.

Scientists at USGS, San Francisco Estuary Institute, and Moss Landing Marine Laboratory collaborated with the state's Surface Water Ambient Monitoring Program (SWAMP) and the Bioaccumulation Oversight Group (BOG) to assess mercury exposure to wildlife in California lakes and reservoirs. This project studied grebes (Aechmophrous spp.)—birds which dive underwater to hunt small fish—as a model of how mercury levels are magnified through lake ecosystems from prey to predator. Grebes and other fish-eating wildlife can accumulate mercury in their bodies as they continue to consume mercury-contaminated fish over the course of their lifetime.

This project resulted in a tool for estimating wildlife and sport fish risk to mercury exposure, based on mercury concentrations in prey fish. This quantitative tool can be used to predict mercury concentrations in bird blood, bird eggs, and sport fish—facilitating a feasible alternative for estimating wildlife exposure to mercury, when more comprehensive wildlife sampling is not possible. This tool may have useful applications for the State Water Board’s Total Maximum Daily Load (TMDL) derivation and for monitoring mercury contamination in California lakes.

 

Related Partners:

Monitoring Mercury's Risk to Wildlife in CA Lakes, Project Partners
Monitoring Mercury's Risk to Wildlife in California Lakes—Project Partner Logos(Public domain.)

 

Project Background—Assessing Wildlife Exposure to Mercury

Numerous water bodies in California are listed under the Clean Water Act as being impaired due to mercury contamination. The Surface Water Ambient Monitoring Program (SWAMP), via the Bioaccumulation Oversight Group (BOG), has recently completed statewide surveys of contaminants in sport fish tissue from more than 250 lakes and rivers in California and throughout coastal waters.  

This effort focused on human health issues, but did not include beneficial uses by wildlife. Many fish-eating birds such as grebes, terns, cormorants, and mergansers eat fish smaller than those that were previously sampled, and sport fish mercury concentrations are not always indicative of wildlife exposure to mercury.  

This USGS project developed a tool for estimating wildlife and sport fish risk from mercury exposure based on mercury concentrations in prey fish. This quantitative tool can be used to predict mercury concentrations in fish-eating birds and sport fish, thus facilitating a feasible alternative for adequately estimating wildlife exposure when more comprehensive wildlife sampling is not possible.

 

Project Objectives and Sampling Plan

The research team used western grebes (Aechmophorus occidentalis) and Clark’s grebes (Aechmophorus clarkii) as the index of mercury exposure to wildlife in California lakes. Specifically, the team sampled grebes, prey fish, and sport fish simultaneously at 25 lakes throughout California during the spring and summer of 2012 and 2013 when breeding birds are particularly vulnerable to mercury-induced reproductive impairment. 

Altogether, the team sampled and analyzed mercury concentrations in tissue from 354 grebes at 25 lakes, 101 grebe eggs at 7 lakes, 505 prey fish comprised of 14 species from 25 lakes, and 230 sport fish comprised of 5 species from 24 lakes. 

Using these data, the team evaluated which environmental variables influenced mercury concentrations in grebe blood, grebe eggs, and sport fish. For each of these tissues, the team built a set of candidate models based on potential predictor variables describing the 1) specific animal tissue, 2) lake attributes, and 3) mercury concentrations in prey fish.  

Specifically, the team addressed three management questions:

  1. Does methylmercury pose significant risks to aquatic life in a representative sample of California lakes and reservoirs?
  2. Can a correlational approach be applied on a statewide basis to estimate risks to birds?
  3. What are appropriate water-quality monitoring requirements to address methylmercury exposure in wildlife?

 

Project Map

Map of field sampling locations for Mercury Risk in Lakes project
Map showing location of the 25 lakes and reservoirs where grebes, sport fish, and prey fish were collected for mercury analyses of blood, eggs, and tissue, California, 2012 ̶ 13. Topography layer by U.S. National Park Service.(Public domain.)

 

Final Report and Predictive Model

  • Navigate to the "Publications" tab to download the report and model.

 

Predictive Tool for Managers

Using the data gathered, the team built predictive equations for each tissue type using linear mixed-effect models, Akaike’s Information Criterion, and model-averaging to evaluate which variables influenced mercury concentrations in grebe blood, grebe eggs, and sport fish.

The team then built a predictive tool built in Microsoft Excel that can facilitate calculations by natural resource managers using these predictive equations (see link to file entitled “USGS Wildlife and Sport Fish Risk Estimator Tool Final” in data and tools tab).

Tool users will enter mercury concentrations in prey fish, date sampled, and the specific lake’s attributes, and this Excel tool will then predict mercury concentrations in grebe blood, grebe eggs, and sport fish. Furthermore, the tool uses these estimated values to assess the relative risk to the animal by comparing the estimated mercury concentrations to published toxicity benchmarks. A tutorial is included in the tool.

  • To download the tool, navigate to the "Data and Tools" tab

This project was supported by the California State Water Resources Control Board's Surface Water Ambient Monitoring Program (SWAMP) through the Bioaccumulation Oversight Group (BOG).