Determination of Total Mercury in 

Fillets of Sport Fishes Collected from 

Folsom and New Melones Reservoirs, 

California, 2004

Prepared in cooperation with the U.S. Bureau of Reclamation

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007–1077

Determination of Total Mercury in Fillets of Sport Fishes 

Collected from Folsom and New Melones Reservoirs, 

California, 2004 

By Thomas W. May and William G. Brumbaugh 

Prepared in cooperation with the U.S. Bureau of Reclamation 

Open-File Report 2007–1077 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007 

For product and ordering information: 

World Wide Web: http://www.usgs.gov/pubprod 

Telephone: 1-888-ASK-USGS 

For more information on the USGS—the Federal source for science about the Earth, 

its natural and living resources, natural hazards, and the environment: 

World Wide Web: http://www.usgs.gov 

Telephone: 1-888-ASK-USGS 

Suggested citation:

May, T.W., and Brumbaugh, W.G., 2007, Determination of total mercury in fillets of sport fishes collected from Folsom 

and New Melones Reservoirs, California, 2004: U.S. Geological Survey Open-File Report 2007–1077, 8 p. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 
Contents 

Abstract.....................................................................................................................................................................................................................1 

Introduction...............................................................................................................................................................................................................1 

Sample History .........................................................................................................................................................................................................2 

Methods.....................................................................................................................................................................................................................2

Homogenization and Lyophilization..................................................................................................................................................................2 

Instrumental Analysis and Data Reporting .....................................................................................................................................................2

Results and Discussion...........................................................................................................................................................................................3

Quality Control..........................................................................................................................................................................................................3 

References Cited......................................................................................................................................................................................................6

Tables 

1. Total mercury concentrations in fillets of sport fishes from Folsom Reservoir, California, 2004...............................................4 

2. Total mercury concentrations in fillets of sport fishes from New Melones Reservoir, California, 2004 .................................5 

3. Total mercury concentrations in dry tissue powders submitted with whole-body fish samples, 2004 ....................................6 

Conversion Factors 

Multiply By To obtain 

Mass 

gram (g) 0.03527 ounce (oz) 

milligram (mg) .000035 ounce (oz) 

Length 

millimeter (mm) 0.0394 inches (in) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: 

°F=(1.8×°C)+32 

Concentrations of chemical constituents in solid materials are given in microgram per gram (μg/g). 

iii
iv
Determination of Total Mercury in Fillets of Sport Fishes 

Collected from Folsom and New Melones Reservoirs, 

California, 2004 

By Thomas W. May and William G. Brumbaugh 

Abstract 

This report presents the results of a study by the U.S. Geological Survey, done in cooperation 

with the U.S. Bureau of Reclamation, to determine mercury concentrations in selected sport fishes from 

Folsom and New Melones Reservoirs in California. Fillets were collected from each fish sample, and 

after homogenization and lyophilization of fish fillets, mercury concentrations were determined with a 

direct mercury analyzer utilizing the process of thermal combustion-gold amalgamation atomic 

absorption spectroscopy. Mercury concentrations in fish fillets from Folsom Reservoir ranged from 0.09 

to 1.16 micrograms per gram wet weight, and from New Melones Reservoir ranged from 0.03 to 0.94 

microgram per gram wet weight. Most of the fish fillets from Folsom Reservoir (87 percent) and 27 

percent of the fillets from New Melones Reservoir exceeded the U.S. Environmental Protection 

Agency’s fish consumption advisory of 0.30 microgram per gram wet weight. 

Introduction 

Mercury is a byproduct of the extensive gold mining performed in the Sierra Nevada Mountains 

of California in the nineteenth century. After being discarded by miners, the residual mercury was 

gradually released into the downstream environment, including Folsom and New Melones Reservoirs. 

These two reservoirs are created by Folsom and New Melones Dams, which are managed by the U.S. 

Bureau of Reclamation (USBR) for multiple uses, including water supply and recreation. Folsom 

Reservoir is on the American River northeast of Sacramento, whereas New Melones Reservoir is 

located on the Stanislaus River 60 miles upstream from the confluence of the San Joaquin River. Both 

of these reservoirs support sport fisheries for several species; the concern is that mercury residues now 

approach or exceed guidelines for human consumption. 

The U.S. Geological Survey (USGS) conducted a study in cooperation with the USBR to 

determine the concentrations of mercury in selected sport fishes from Folsom and New Melones 

Reservoirs. The State of California and USBR also are cooperating to address this concern. The 

California Department of Fish and Game and USBR personnel conducted all fish collections. The 

USBR will use the data from this study to make future management decisions concerning fish harvests 

from these reservoirs. The data will be provided to state and local agencies responsible for evaluating 

the potential risk to the public from fish consumption. Fish health advisories will be issued if necessary. 

1
Sample History 

A shipment of 36 whole-body fish and 4 dry tissue powder samples collected by USBR from 

Folsom Reservoir were received by the USGS on August 3, 2004. Upon receipt, the samples were 

assigned the USGS batch number 1036 and USGS identification numbers 32439–32478. A second 

shipment arrived on August 12, 2004, from USBR scientists, which consisted of 4 whole-body fish from 

Folsom Reservoir, 33 whole-body fish from New Melones Reservoir, and 5 dry tissue powder samples. 

This shipment was assigned USGS batch number 1041 and USGS identification numbers 32530–32571. 

A third shipment from USBR consisted of 6 whole-body fish from Folsom Reservoir, 12 whole-body 

fish from New Melones Reservoir, and 3 dry tissue powder samples. This final shipment was assigned 

USGS batch number 1054 and USGS identification numbers 32740–32760. The USBR requested that 

the USGS conduct a survey of mercury contamination in the edible portions (fillets) of selected sport 

fishes from the reservoirs. 

Methods 

Homogenization and Lyophilization 

Filleting of whole-body fish samples was conducted with a titanium knife. A fillet was collected 

from one side of all fish, except for one small sample, which had fillets removed from both sides to 

provide a more suitable biomass. The method used to homogenize fish fillets was based on the size of 

the fillet sample. Fillet samples > (greater than) 200 g (grams) were processed through a Hobart® meat 

grinder, whereas fillet samples from 100 to 200 g were homogenized using a Kitchen Aid®

 blender with 

an attached meat processor unit. Fillet samples < (less than) 100 g were minced with a titanium knife. 

Homogenized samples were lyophilized with a Virtis Genesis®

 35EL freeze dryer and percent moisture 

was determined as part of the lyophilization procedure. For dry tissue powders, residual percent 

moisture was determined by heating a 1-gram subsample for 4 hours at 90 to 95 o

C (Celsius) in a gravity 

convection oven. Once dried, fillet samples were further homogenized using a Bamix® Mixer/Blender. 

All dried samples were stored in glass vials in a desiccator. 

Instrumental Analysis and Data Reporting

Mercury was determined with a direct mercury analyzer. With this method, a dried fish sample 

of approximately 50 to 100 mg (milligrams) was combusted in a stream of oxygen. All mercury in the 

sample was volatilized and trapped by amalgamation on a gold substrate and thermally desorbed and 

quantitated by atomic absorption spectrophotometry (U.S. Environmental Protection Agency, 2003). 

The entire sequence was conducted with a Milestone Direct Mercury Analyzer (DMA-80) equipped 

with an automated sample carousel. Duplicate determinations were conducted for each sample and the 

mean of the two analyses was reported; however, if the relative percent difference (RPD) among 

duplicates exceeded 20 percent, an additional analysis was performed and the concentration expressed 

as the mean of all three analyses. The concentrations of mercury measured in dried fillet samples was 

converted to wet weight for reporting based on moisture contents determined by lyophilization, but 

concentrations of samples received as dry powders were reported “as received.” Based on our historical 

measurements, residual moisture of homogenized, desiccator-stored fish fillet tissue typically is between 

2 and 3 percent, but we made no attempt to apply a correction for residual moisture of individual dry 

2
samples because the mercury analyzer is calibrated with dried certified reference tissues having similar 

ranges of moisture content. The uncertainty of reported wet-weight concentrations, potentially because 

of moisture differences among the individual dried unknown samples and the certified reference 

samples used for the calibration, is expected to be considerably less than the overall method uncertainty 

[about ± (plus or minus) 5 to 10 percent for mean concentrations well above the quantitation limit]. 

Furthermore, the uncertainty associated with moisture variation among dry sample determinations 

should be no greater than the uncertainty associated with moisture variation among fresh or frozen fish 

fillet sample analyses. 

Results and Discussion 

Percent moisture and mean concentrations of total mercury measured in fish fillet samples 

collected from various sport fishes of Folsom Reservoir are presented in table 1. Percent moisture in 

fillets ranged from 75.0 to 85.4 percent and averaged 79.4 percent. Mercury concentrations ranged from 

0.09 to 1.16 μg/g (micrograms per gram) wet weight and averaged 0.58 μg/g wet weight. Out of 46 fish 

fillet samples, 40 (87 percent of the sample set) contained mercury concentrations exceeding the U.S. 

Environmental Protection Agency fish consumption advisory of 0.30 μg/g wet weight (U.S. 

Environmental Protection Agency, 2001). 

Percent moisture in fillets of fish from New Melones Reservoir ranged from 66.4 to 81.8 percent 

and averaged 75.9 percent (table 2). However, all of the salmon submitted from New Melones 

Reservoir (32747–32754; 32756–32759) were in poor condition; for example, the flesh appeared to be 

partially decomposed and the moisture contents for these particular fish (66.4 to 73.3 percent) were 

lower than normal. Adjustment to a normal moisture content might be preferable if the mercury 

concentrations in these fillets are to reflect “fresh” fish samples. Total mercury concentrations in fish 

fillet samples from New Melones Reservoir (table 2) ranged from 0.03 to 0.94 μg/g wet weight and 

averaged 0.23 μg/g wet weight. Out of 45 fish fillet samples, 12 (27 percent of the sample set) exceeded 

the 0.30 μg/g fish consumption advisory guideline (U.S. Environmental Protection Agency, 2001). 

Percent moisture and concentrations of total mercury in submitted dry tissue are presented in 

table 3. Percent moisture ranged from 3.1 to 4.3 percent. Mercury concentrations ranged from 2.83 to 

2.94 μg/g dry weight and 5.09 to 5.32 μg/g dry weight. As mentioned earlier, mercury concentrations 

were not corrected for residual moisture because the DMA-80 was calibrated using dried reference 

tissue powders with comparable residual moisture values. 

Quality Control 

The samples were handled in five groups or blocks through the instrumental analysis. Quality 

control included blanks, replicates, pre-combustion spikes, and tissue reference materials. During the 

instrumental run, additional quality control included independent calibration verification checks. 

For each group or block of samples, an independent calibration verification sample [National 

Research Council Canada (NRCC) DOLT-2] was analyzed at the beginning and end of the instrumental 

run to confirm the calibration status of the DMA-80 system; each measured calibration sample was 

within ±10 percent of the certified concentration. Four reference tissues were analyzed for mercury: 

NRCC DORM-2 [n=9 (9 samples)], National Institute of Standards and Technology (NIST) RM50 

(n=5), International Atomic Energy Agency (IAEA) 407 (n=5), NRCC DOLT-2 (n=5); recoveries of 

mercury were within certified or recommended ranges. Method precision can be estimated either from 

the RPD from the duplicate analysis of tissue samples or as percent relative standard deviation (RSD) 

3
Table 1. Total mercury concentrations in fillets of sport fishes from Folsom Reservoir, California, 2004.

[USGS, U.S. Geological Survey; ID, identification; n, number; μg/g, micrograms per gram wet weight] 

 

Mean total 

USGS ID Field/lab ID Percent mercury 

number number moisture n (μg/g) 

32439 FLF 001 78.0 2 0.51 

32440 FLF 002 80.3 2 0.47 

32441 FLF 003 83.4 2 0.65 

32442 FLF 004 81.7 2 0.56 

32443 FLF 005 80.6 2 0.37 

32444 FLF 006 81.5 2 0.59 

32445 FLF 007 77.8 2 0.50 

32446 FLF 008 81.2 2 0.49 

32447 FLF 009 80.0 2 0.51 

32448 FLF 010 80.7 2 1.04 

32449 FLF 011 75.9 2 0.71 

32450 FLF 012 77.4 2 0.94 

32451 FLF 013 78.7 2 0.56 

32452 FLF 014 79.4 2 0.36 

32453 FLF 015 75.9 2 0.59 

32454 FLF 016 80.3 2 0.38 

32455 FLF 017 77.2 2 1.02 

32456 FLF 018 79.0 2 0.65 

32457 FLF 019 79.4 2 0.43 

32458 FLF 020 76.6 2 0.80 

32459 FLF 021 80.4 2 0.38 

32460 FLF 022 78.0 2 0.64 

32461 FLF 023 77.7 2 0.87 

32462 FLF 024 76.2 2 0.83 

32463 FLF 025 82.0 2 0.18 

32464 FLF 026 84.4 2 0.09 

32465 FLF 027 85.4 2 0.33 

32466 FLF 028 80.3 2 0.51 

32467 FLF 029 78.2 2 0.60 

32468 FLF 030 78.5 2 0.56 

32469 FLF 031 82.3 2 0.47 

32470 FLF 032 81.2 2 0.98 

32471 FLF 033 79.6 2 0.92 

32472 FLF 034 76.5 2 1.16 

32473 FLF 035 78.7 2 0.74 

32474 FLF 036 75.3 2 0.45 

32567 FLF 038 78.7 2 0.85 

32568 FLF 039 78.4 2 1.00 

32569 FLF 040 75.0 2 0.47 

32570 FLF 041 83.7 2 0.12 

32740 FLF 043 78.2 2 0.10 

32741 FLF 044 80.0 2 0.46 

32742 FLF 045 83.2 2 0.91 

32743 FLF 046 75.9 2 0.81 

32744 FLF 047 77.8 2 0.11 

32745 FLF 048 82.2 2 0.22 

4
Table 2. Total mercury concentrations in fillets of sport fishes from New Melones Reservoir, California, 2004. 

[USGS, U.S. Geological Survey; ID, identification; n, number; μg/g, micrograms per gram wet weight] 

 

Mean total 

USGS ID Field/lab ID Percent mercury 

number number moisture n (μg/g) 

32530 NMRF 001 78.8 2 0.24 

32531 NMRF 002 75.6 2 0.11 

32532 NMRF 003 78.1 2 0.28 

32533 NMRF 004 78.5 2 0.09 

32534 NMRF 005 77.5 2 0.08 

32535 NMRF 006 76.9 2 0.15 

32536 NMRF 007 76.8 3 0.19 

32537 NMRF 008 75.5 2 0.11 

32538 NMRF 009 79.0 2 0.06 

32540 NMRF 011 77.5 2 0.49 

32541 NMRF 012 79.7 2 0.31 

32542 NMRF 013 78.3 2 0.45 

32543 NMRF 014 81.8 2 0.94 

32544 NMRF 015 79.7 2 0.54 

32545 NMRF 016 79.6 2 0.60 

32546 NMRF 017 80.2 2 0.40 

32547 NMRF 018 79.0 2 0.33 

32548 NMRF 019 78.9 2 0.34 

32550 NMRF 021 72.2 2 0.09 

32551 NMRF 022 79.0 2 0.05 

32552 NMRF 023 71.1 3 0.16 

32553 NMRF 024 73.7 2 0.05 

32554 NMRF 025 80.0 2 0.18 

32555 NMRF 026 70.6 2 0.18 

32556 NMRF 027 78.3 2 0.12 

32557 NMRF 028 78.7 2 0.05 

32558 NMRF 029 80.6 2 0.03 

32560 NMRF 031 80.1 2 0.68 

32561 NMRF 032 80.3 2 0.41 

32562 NMRF 033 78.7 2 0.27 

32563 NMRF 034 78.8 2 0.27 

32564 NMRF 035 78.7 2 0.53 

32565 NMRF 036 80.1 2 0.24 

32747 NMRF 038 72.7 2 0.08 

32748 NMRF 039 72.2 2 0.09 

32749 NMRF 040 69.8 2 0.11 

32750 NMRF 041 66.4 2 0.13 

32751 NMRF 042 67.9 2 0.12 

32752 NMRF 043 69.9 2 0.14 

32753 NMRF 044 68.3 2 0.11 

32754 NMRF 045 67.9 3 0.11 

32756 NMRF 047 71.6 2 0.09 

32757 NMRF 048 71.4 2 0.11 

32758 NMRF 049 73.3 2 0.07 

32759 NMRF 050 72.4 2 0.09 

5
Table 3. Total mercury concentrations in dry tissue powders submitted with whole-body fish samples, 2004.

[USGS, U.S. Geological Survey; ID, identification; n, number; μg/g, micrograms per gram dry weight] 

 

Mean total 

USGS ID Field/lab ID Percent mercury 

number number moisture n (μg/g) 

32475 FLF 005A 3.8 2 2.94 

32477 FLF 025A 4.3 2 2.84 

32549 NMRF 020 4.1 2 2.83 

32571 FLF 042 3.1 2 2.88 

32760 NMRF 051 4.0 2 2.87 

32476 FLF 015A 4.1 2 5.29 

32478 FLF 037 3.9 2 5.15 

32539 NMRF 010 3.7 2 5.11 

32559 NMRF 030 3.5 2 5.14 

32566 NMRF 037 3.2 2 5.21 

32746 FLF 049 4.1 2 5.09 

32755 NMRF 046 3.5 2 5.32 

from triplicates. The overall mean RPD ± the standard deviation for 103 duplicate pairs of fish fillets 

(n=91) and fish powders (n=12) was 4.7 percent ±4.5. RPDs were <10 percent for all samples except 

seven, the latter ranging from 10.3 to 23 percent. Samples with RPDs >20 percent (n=3) were analyzed 

again to obtain a triplicate analysis. Percent RSDs from triplicate analyses of these and other arbitrarily 

selected samples (n=13) used for spike recovery analysis were ≤13 percent. Percent recovery of 

methylmercury hydroxide from pre-combustion tissue spikes (n=10) ranged from 106 to 125 percent, 

and averaged 113 percent. Although all spike recoveries were above 100 percent, the mean recovery 

was well within quality control protocol target limits (80 to 120 percent). Moreover, spike recoveries are 

not necessarily the best accuracy indicator for the DMA-80 method because the instrumental response 

for liquids is sometimes slightly different than for solids. Blank equivalent concentrations (BECs) for 

total mercury exceeded method detection limits for three of the five sample blocks, but the lowest 

sample concentration (0.03 μg/g wet weight) was about 200 times the highest BEC concentration 

(0.00014 μg/g wet weight). The instrument detection limit was 0.0053 μg/g dry weight mercury. The 

method detection limits ranged from 0.00004 to 0.00021 μg/g wet weight mercury, and limits of 

quantitation ranged from 0.0014 to 0.0077 μg/g wet weight. Overall, the quality control was within 

acceptable limits as specified by the USGS. 

References Cited 

U.S. Environmental Protection Agency, 2001, Water quality criterion for protection of human health—

Methylmercury: Washington, D.C., U.S. Environmental Protection Agency, Office of Water, EPA823-R-01-001, 269 p. 

U.S. Environmental Protection Agency, 2003, Draft update IVA of SW-846, 7000 series, Method 7473, 

Mercury in solids and solutions by thermal decomposition, amalgamation, and atomic absorption 

6
spectrophotometry: accessed July, 2005 at URL 

http://www.epa.gov/epaoswer/hazwaste/test/up4a.htm#7_series.

7
For more information concerning the research described 

in this report, contact: 

U.S. Geological Survey 

Columbia Environmental Research Center 

4200 New Haven Road 

Columbia, MO 65201 

(573) 875–5399 

http://www.cerc.usgs.gov

8
9
May and Brumbaugh—Determination of Total Mercury in Fillets of Sport Fishes Collected from Folsom and —Open-File Report 2007–1077

New Melones Reservoirs, California, 2004