Results of TSF’s Angler Mercury Exposure study published in CA Rural Health Advocate

The following article, by The Sierra Fund’s Science Director Carrie Monohan, was published by the CA State Rural Health Association in their June edition of the Rural Health AdvocateThe final report from this study will be available on TSF’s website shortly!   


Fish Consumption from Lakes and Streams in the Sierra – A Rural Health Problem

Written by: Carrie Monohan, Ph.D., Science Director, The Sierra Fund

On a warm afternoon in May, a local geologist and volunteer survey administrator approached a group of Hmong men and women fishing at the dam of a low elevation Sierra Foothills reservoir.  The Hmong anglers had a stringer of impressive sized largemouth bass that they indicated they were going to take home and eat.  One man agreed to participate in the ten minute survey, and answered a series of questions about where he fished, what he caught, and how much fish he ate.  Later, based on his answers, his mercury exposure was calculated to be 21.94 micrograms of methylmercury per day, above California’s safe consumption levels even for adult men. 

Many questions remain about the exposure of rural people to mercury from eating fish they catch.  Are people aware of high mercury levels in the fish they catch?  Are people, and especially sensitive populations of women and children, consuming enough mercury in fish they catch to affect their health?  In difficult financial times, are people increasingly turning to fishing as a way to reduce grocery bills?  Are low-income and ethnic people traveling to rural areas to fish because they think the fish are less contaminated?  These questions are especially pertinent in the Sierra Nevada, a 25 million acre region of rural California where 13 million pounds of mercury was lost to the environment over 150 years ago during intensive gold mining.

In response, over the summer of 2009 The Sierra Fund and Friends of Deer Creek, nonprofit organizations in Nevada City, California, conducted the Gold County Angler Survey, a pilot study to assess mercury exposure from sport fish consumption in the Sierra Nevada.   Results of this pilot study indicate that some rural people are being exposed to dangerous levels of mercury by eating fish they catch from Sierra waterways, and that their awareness of the associated health effects is low.  The study illustrates the need for posting areas with existing fish advisories, increased efforts to educate fishing populations, and more extensive surveying to characterize mercury exposure of rural people in the Sierra. 

The purpose of the 2009 Gold Country Angler Survey was to learn whether people who fish in Sierra Nevada waters eat fish they catch and if they are being exposed to mercury.  The survey targeted anglers who fish in waterways that were once hydraulically mined to characterize fishing activities, fish consumption patterns, approximate mercury exposure, and health hazard awareness. 

Trained volunteers conducted the survey questionnaire at 12 locations, making approximately 48 visits to fishing locations or cultural events, spending approximately 78 hours in the field.  A total of 69 surveys were completed, the majority obtained from major reservoirs that are easily accessible from both Nevada City/Grass Valley area, as well as rural and low-income communities in the Sacramento Valley. 

Fish Consumption
 Nearly half (45%) of the anglers reported eating fish they catch, and over half of those (58%) provide these fish to their families for consumption.  Fishermen who provide sport fish for consumption to children under 18, women of child-bearing age, and pregnant women may be exposing these sensitive populations to health risks from mercury. 

The most popular fish eaten were bass and trout, which tend to have the highest mercury concentrations and are the subject of fish consumption advisories.  Almost all survey participants that eat sport fish (74%) had eaten trout in the previous 30 days.  A significant number of respondents reported eating one or all three kinds of bass (largemouth (41%), smallmouth (35%), and striped (24%)).  Other popular species included kokanee salmon (22%), catfish (17%), and crappie (17%).

Health Hazard Awareness
Although 90% of survey participants knew that there was health warnings about eating fish, Sierra anglers generally lacked awareness of the specific information needed to make informed decisions about local fish consumption.  Specific information in the form of posted warnings was not apparent at most fishing locations, even if the water body was the subject of existing state fish consumption advisory. 

Results show that 40% of survey participants consider healthcare providers the most trusted source of health information. Other sources included family members, TV, and the fishing regulation handbook.  This finding is significant, since a 2007 survey of rural Sierra health clinics found that not one of the 13 surveyed clinics used an Environmental Health History form to learn about fish consumption patterns, and not one provided information about mercury fish (whether bought or caught locally) as a part of their maternal/infant health program (The Sierra Fund 2008). 

Methylmercury Exposure
In order to learn whether anglers were eating fish within safe mercury exposure guidelines, the approximate exposure of each survey participant was calculated based on their answers to questions about fish consumption in the past 30 days.  The number of times the participant reported eating a particular species of sport fish in the last 30 days was multiplied by reported portion sizes and the average mercury concentration determined for that species.  This calculation was performed for each species of fish the participant reported eating.  Resulting values were added to achieve a cumulative total.  The result was the approximate amount of methlymercury consumed in micrograms per day. 

In the graph, each bar represents the calculated exposure of each survey respondent.  The highest individual level of exposure was 21.94 micrograms of methylmercury per day, from a Hmong man between the ages of 39 and 45 fishing at Lake Wildwood, who reported eating largemouth bass at least four times a month.  The second highest level of exposure was 17.12 micrograms of methylmercury per day, from a Native American man over the age of 45.  He was fishing in the South Yuba River and reportedly ate trout and salmon, each eight times in the last month.  The third and fourth highest levels were both males over the age of 45 who reported fishing at Lake Englebright for largemouth bass, smallmouth bass, crappie and rainbow trout which they both ate two to three times a month. 

The approximate exposure for each participant is conservative, since it was calculated using fish mercury levels from Central Valley samples.  Data from the Central Valley Regional Water Quality Control Board (2008) were used because they were more complete than fish data for Sierra reservoirs.  However, in many instances, Sierra fish have higher mercury levels than those indicated by the CWRWQCB.  For example, the average for largemouth bass reported by the CVRWQCB was 0.774ppm, while the highest value for largemouth bass in a Sierra water body (Combie Reservoir) was 0.907ppm (May et al.  1999).  For Sierra Brown Trout, the average mercury concentration was 0.121ppm (May et al.); more than double the value of 0.061ppm reported by CVRWQCB (2008) for rainbow trout.  If you use the fish mercury level for brown trout to re-calculate the mercury exposure level for the top two sport fish consumers it increases their exposure level from 17 and 22 µg Hg /day to 22 and 24 µg Hg /day respectively, which are both above the OEHHA recommended level of 21 µg Hg /day. This data comparison indicates that more fish data are needed from Sierra water bodies to accurately calculate the methylmercury exposure from fish consumption. 

The exposure of each participant was compared to Office of Health Hazard Assessment (OEHHA) recommended safe levels of methylmercury exposure :   0.1 µg of mercury/kg of body weight per day for women aged 18 to 45 and children under 17, and 0.3 µg or mercury/kg per day for women over 45 and men (OEHHA 2008).  Separate safe exposure levels for men and women of child bearing age and children are considered because OEHHA has determined that developing fetuses and children are more sensitive to the harmful effects of methylmercury than adults.  The safe level of exposure depends on each individual’s body weight.  For this survey, adult participants’ body weight was assumed to be 70kg (154lbs). 

This conservative evaluation suggests that the majority of survey respondents were not exposed to dangerous levels of methylmercury when eating locally-caught fish; however the exposure potential remains high.  If the women and men were less than 154lbs, they may be exposed to methylmercury above OEHHA recommended levels.  Furthermore, exposure to methylmercury through consumption of commercial fish was not considered in this calculation, therefore participants’ actual exposure may be higher still.  Future surveys should quantify commercial fish consumption levels in addition to sport fish consumption. 

The study results indicate that an expanded survey is warranted to capture a broader population and better understand the risks associated with eating sport fish from Sierra water ways.  It is also important to collect fish data from the water bodies in the Sierra Nevada where people are fishing to accurately capture the exposure potential. 
In the mean time, healthcare providers need to fill a key education role by providing information about sport fish consumption to their patients.  Finally, areas that have fish advisories need to be clearly posted so that people who are consuming fish are aware of the potential health risks.  


Alpers et al.  United States Geological Service.  2000 (updated March 2005).  Fact Sheet: Mercury Contamination from Historic Gold Mining in California. 
Agency for Toxic Substances and Disease Registry (ATSDR).  1999.  Toxicological profile for mercury (update).  Prepared by Research Triangle Institute under contract no.  205-93-0606. Public Health Service, U.S.  Department of Health and Human Services. 
Bloom, N.S.  1992.  On the chemical form of mercury in edible fish and marine invertebrate tissue.  Can.  J.  Fish.  Aquat.  Sci.  49(5):1010-1017. 
Cal/EPA OEHHA.  June 2008.  Development of Fish Contaminant Goals and Advisory Tissue Levels for Common Contaminants in California Sport Fish: Chlordane, DDTs, Dieldrin, Methylmercury, PCBs, Selenium, and Tosaphene.
Central Valley Regional Water Quality Control Board (CVRWQCB).  2008.  Sacramento-San Joaquin Delta Estuary TMDL for Methylmercury, Draft Staff Report.  Central Valley Regional Water Quality Control Board.
Churchill, R.K.  2000.  Contributions of Mercury to California’s Environment from Mercury and Gold Mining Activities – Insights from the Historical Record.  Extended abstracts for the U.S.  EPA-sponsored meeting, Assessing and Managing Mercury from Historic and Current Mining Activities. 
Fish Mercury Project.  June 2006.  Report of Fishing Activities in the North Delta and Sacramento River Watershed.
May J.T., R.L.  Hothem, C.N.  Alpers, and M.A.  Law.  1999.  Mercury Bioaccumulation in Fish in a Region Affected by Historic Gold Mining: The South Yuba River, Deer Creek and Bear River Watersheds.  USGS 00-367.
The Sierra Fund.  March 2008.  Mining’s Toxic Legacy:  An Initiative to Address Mining Toxins in the Sierra Nevada. 

About the Author:
Dr. Carrie Monohan earned her Ph.D. in Forest Engineering and Hydrology in 2004 from the University of Washington, Seattle.  Her dissertation work addressed the relationship between water quality in agricultural streams and diminishing salmon habitat.  Throughout her graduate program, she was a research assistant to the Northwest Fisheries Science Center, National Oceanic and Atmospheric Administration (NOAA).  Other notable recent positions include Senior River Scientist for the Natural Heritage Institute and project manager and lead scientist for the EPA Brownfields Community Wide Assessment in Nevada City.  Carrie has worked as a consultant to The Sierra Fund since 2007, and was hired as staff Science Director in 2010.  Carrie recently received the Center for Whole Communities Fellowship award, 2042 Today: Young Leaders Re-Imagining Conservation.