Determination of Total Mercury in 

Fillets of Sport Fishes Collected from 

Folsom Reservoir, California, 2006

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

U.S. Department of the Interior

U.S. Geological Survey

Open-File Report 2007–1078

Determination of Total Mercury in Fillets of Sport Fishes 

Collected from Folsom Reservoir, California, 2006 

By Thomas W. May and William G. Brumbaugh 

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

Open-File Report 2007–1078 

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 

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Suggested citation:

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

Reservoir, California, 2006: U.S. Geological Survey Open-File Report 2007–1078, 6 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 

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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..........................................................................................................................................................................................................4 

References Cited......................................................................................................................................................................................................5

Tables 

1. Total mercury concentrations in fillets of sport fishes from Folsom Reservoir, California, 2006...............................................3 

2. Total mercury concentrations in dry tissue powders submitted with whole-body fish samples, 2006 ....................................4 

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). 

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Determination of Total Mercury in Fillets of Sport Fishes 

Collected from Folsom Reservoir, California, 2006 

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 Reservoir 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 fillets ranged from 0.031 to 0.20 micrograms per gram wet 

weight in rainbow trout (Oncorhynchus mykiss) samples and 0.071 to 0.16 micrograms per gram wet 

weight in bluegill (Lepomis macrochirus) samples. Mercury concentration was 0.98 microgram per 

gram wet weight in a single spotted bass (Micropterus punctulatus) sample, which was the only one in 

the sample set which 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 that of the American River. Folsom 

Reservoir is on the American River northeast of Sacramento and supports recreational fishing for 

several cold and warm water fish species, including rainbow trout (Oncorhynchus mykiss), brown trout 

(Salmo trutta), bass, catfish, crappie, perch, and bluegill (Lepomis macrochirus). Created by Folsom 

Dam, this reservoir is managed by the California Department of Parks and Recreation under an 

agreement with the U.S. Bureau of Reclamation (USBR). 

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 Reservoir. The California 

Department of Fish and Game (CDFG), along with USBR personnel, conducted all fish collections. 

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

harvests from Folsom Reservoir.

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Sample History 

A shipment of 14 whole-body fish and 4 dry tissue powder samples collected by USBR was 

received by the USGS on June 28, 2006. The fish samples were collected by line fishing and consisted 

of 13 rainbow trout and 1 spotted bass (Micropterus punctulatus). Upon receipt, the samples were 

assigned USGS batch number 1264 and USGS identification numbers 37049–37066. A second 

shipment arrived on August 16, 2006 that consisted of nine whole-body bluegill sunfish and two dry 

tissue powder samples. This shipment was assigned USGS batch number 1285 and USGS identification 

numbers 37724–37734. Before receipt by the USGS, all whole-body fish samples had organs removed 

(liver, spleen, and one kidney) by the CDFG for a separate study. The USBR requested that the USGS 

determine the fork length on all fish and total length on rainbow trout, then remove a fillet sample, and 

determine percent moisture and total mercury on all submitted samples. 

Methods 

Homogenization and Lyophilization 

All methods and procedures employed followed approved USGS Standard Operating 

Procedures. Filleting of the larger fish was conducted with a titanium knife; smaller fish were filleted 

using a ceramic knife. A fillet was collected from one side of all fish, except for bluegill, which had 

fillets removed from both sides to provide a more suitable biomass. All fillet samples were minced with 

a titanium knife. Homogenized samples and tissue reference materials were lyophilized with a Virtis 

Genesis® 35EL freeze dryer, and percent moisture was determined as part of the lyophilization 

procedure. Once dried, fillet samples were further homogenized by crushing with a glass rod in a glass 

sample vial. 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 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 mercury concentrations 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 (tissue reference materials) were reported “as 

received.” Residual moisture of homogenized, desiccator-stored fish fillet tissue typically is between 3 

and 5 percent. No attempt was made to apply a correction for residual moisture of individual dry 

samples because the mercury analyzer was 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 method 

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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 

Fork length, total length, 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 76.7 to 81.1 percent and averaged 79.1 percent. Mercury 

concentrations in rainbow trout samples ranged from 0.031 to 0.20 μg/g (micrograms per gram) wet 

weight and averaged 0.099 μg/g. In bluegill samples, mercury concentrations ranged from 0.071 to 0.16 

μg/g wet weight and averaged 0.11 μg/g. The highest mercury concentration was detected in the single 

spotted bass sample (0.98 μg/g wet weight), which was well above the U.S. Environmental Protection 

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

2001). This was the only fish sample that exceeded this consumption advisory guideline. 

Table 1. Total mercury concentrations in fillets of sport fishes from Folsom Reservoir, California, 2006.

[USGS, U.S. Geological Survey; ID, identification; mm, millimeter; n, number; μg/g, micrograms per gram wet weight; --, no data] 

 

Fork Total Mean total 

USGS ID Field/lab length length Percent mercury 

number ID Species (mm) (mm) moisture n (μg/g) 

37050 FLF051 Rainbow trout 335 365 81.1 2 0.094 

37051 FLF052 Rainbow trout 314 338 78.4 2 0.050 

37052 FLF053 Rainbow trout 398 405 79.1 2 0.20 

37053 FLF054 Rainbow trout 307 337 79.9 2 0.11 

37054 FLF055 Rainbow trout 265 280 77.0 2 0.082 

37055 FLF056 Rainbow trout 302 315 78.0 2 0.031 

37056 FLF057 Rainbow trout 273 288 77.8 2 0.092 

37057 FLF058 Rainbow trout 308 325 76.7 2 0.13 

37060 FLF061 Rainbow trout 310 345 79.5 2 0.095 

37061 FLF062 Rainbow trout 242 254 77.3 2 0.091 

37062 FLF063 Rainbow trout 302 315 78.9 2 0.12 

37063 FLF064 Rainbow trout 285 305 78.6 2 0.082 

37064 FLF065 Rainbow trout 343 360 77.7 2 0.11 

37065 FLF066 Spotted bass 413 435 79.4 2 0.98 

37725 FLF067B Bluegill 125 -- 79.6 2 0.16 

37726 FLF068 Bluegill 153 -- 78.5 2 0.14 

37727 FLF069 Bluegill 149 -- 80.5 2 0.095 

37728 FLF070 Bluegill 124 -- 80.8 2 0.076 

37729 FLF071 Bluegill 130 -- 79.2 2 0.086 

37730 FLF072 Bluegill 122 -- 79.4 2 0.071 

37731 FLF073 Bluegill 137 -- 80.4 2 0.093 

37732 FLF074 Bluegill 129 -- 80.7 2 0.15 

37733 FLF075 Bluegill 145 -- 80.0 2 0.15 

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Percent moisture and mean concentrations of total mercury in submitted dry tissues are 

presented in table 2. Percent moisture ranged from 4.1 to 8.9 percent. Mercury concentrations ranged 

from 0.5 to 2.93 μg/g dry weight. As mentioned earlier, mercury concentrations were not corrected for 

the residual moisture because the DMA-80 is calibrated using dried reference tissue powders with 

comparable residual moisture values. 

Table 2. Total mercury concentrations in dry tissue powders submitted 

with whole-body fish samples, 2006.

[USGS, U.S. Geological Survey; ID, identification; n, number; μg/g, microgram per gram 

as received] 

 

Mean total 

USGS ID Field/lab Percent mercury 

number ID moisture n (μg/g) 

37049 FLF050 8.9 2 0.50 

37058 FLF059 7.4 2 0.51 

37724 FLF067A 7.4 2 0.52 

37734 FLF075A 6.9 2 0.52 

37059 FLF060 4.1 2 2.92 

37066 FLF067 4.3 2 2.93 

Quality Control 

The samples were handled in one group or block 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 the group or block of samples, two independent calibration verification samples [National Research 

Council Canada (NRCC) DOLT-3 and International Atomic Energy Agency (IAEA) 407] were 

analyzed at the beginning and end of the instrumental run to confirm the calibration status of the DMA80 system; each measured calibration sample was within ±10 percent of the certified concentration. 

Five reference tissues were analyzed for mercury: NRCC DORM-2 [n=2 (2 samples)], National Institute 

of Standards and Technology (NIST) RM50 (n=2), IAEA 407 (n=2), NRCC DOLT-3 (n=2), and NIST 

2976 (n=2). Recoveries of mercury were within certified or recommended ranges for all NRCC DORM2 and IAEA 407 materials and for one NIST RM50 material. Recoveries were 95 percent for both NIST 

2976 materials and 99 percent for one NIST RM50 material. Recoveries of mercury in NRCC DOLT-3 

were 102 percent and 104 percent. Method precision can be estimated from the RPD from the duplicate 

analysis of tissue samples or as percent relative standard deviation (RSD) from triplicates. The overall 

mean RPD ± the standard deviation for all fish fillets samples (n=22) was 4.3 percent ±2.2, and for 

tissue powders (n=6) was 4.1 percent ±2.4. Furthermore, RPDs were less than 8 percent for all but one 

sample. One fish fillet sample exhibiting an RPD greater than 20 percent was analyzed again to obtain a 

triplicate analysis. Percent RSDs from triplicate analyses of this and one other arbitrarily selected fish 

fillet sample were 27 percent and 1.8 percent. Percent recovery of methylmercury hydroxide from precombustion tissue spikes (n=4) ranged from 104 to 115 percent and averaged 109 percent. Although all 

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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 indicator of 

accuracy for the DMA-80 method because the instrumental response for liquids is sometimes slightly 

different than for solids. Blank equivalent concentrations (BEC) for total mercury exceeded the method 

detection limit for one of the two determinations, but the lowest sample concentration (0.031 μg/g wet 

weight) was about eight times the highest BEC concentration (0.004 μg/g wet weight). The instrument 

detection limit was 0.0006 μg/g wet weight mercury. The method detection limits were 0.0009 and 

0.005 μg/g wet weight mercury, and the method quantitation limits were 0.003 and 0.017 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 

spectrophotometry: accessed July, 2005 at URL 

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

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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

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May and Brumbaugh—Determination of Total Mercury in Fillets of Sport Fishes Collected from Folsom —Open-File Report 2007–1078

Reservoir, California, 2006