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Huntsville State Park



A recent Texas Parks and Wildlife Department (TPWD) survey of fish from Lake Raven in WalkerCounty revealed elevated levels of mercury in largemouth bass [1]. Consequently, the Texas NaturalResource Conservation Commission (TNRCC) requested that the Texas Department of Health (TDH)evaluate fish from Lake Raven for potential public health hazards to people who eat fish from thisreservoir.

Mercury occurs naturally in the earth's crust but human activity - namely burning fossil fuels - is amajor source of mercury deposited into the environment. Bacteria in soil and water then transforminorganic mercury into methylmercury, an organic form of the element that is highly toxic to humans. Other investigators have reported that most of the mercury in marine life is methylmercury [2]. Seafoodabsorbs and concentrates methylmercury from ambient waters and from other forms of marine life,storing the contaminant primarily in muscle tissues. Methylmercury concentrations in seafood may thusbe many times higher in the tissues of marine vertebrates than in water. Methylmercury is exceptionallytoxic to the immature nervous system, producing adverse effects on the developing brain that vary fromsubtle to severe depending upon the circumstances of exposure [2]. Consumption of mercury-contaminated seafood is typically the main source of exposure to this developmental toxicant [2]. Therefore, regular consumption of fish containing high concentrations of methylmercury, especially byinfants, young children, or pregnant or lactating women, may result in developmental effects on fetuses,infants and children. Other adverse health effects are also possible in both adults and children.

Lake Raven is a picturesque 210-acre reservoir located in Walker County (population 61,768). LakeRaven is eight miles south of Huntsville, Texas (pop. 35,078) [3]. The reservoir lies within the confinesof Huntsville State Park and adjacent to the Sam Houston National Forest in the piney woods area ofeast Texas. Lake Raven is within easy driving distance of Houston, the largest city in Texas, andCollege Station, the home of Texas A & M University. The Civilian Conservation Corps (CCC) createdLake Raven in 1937 by constructing a dam on one of the three major creeks feeding this reservoir. LakeRaven offers a variety of water activities, including fishing and boating. Public access to the lakeincludes fishing piers, shallow banks, trails, and boat-launching ramps. The Inland Fisheries Division(IF) of the TPWD stocks the lake with largemouth bass, catfish, crappie and perch [4]. Recreationalfishing is a very popular pastime at Lake Raven. Subsistence fishing may also occur there. Currentregulations prohibit possession of largemouth bass caught from Lake Raven; instead, fishers may retainthis species only long enough to weigh and measure the specimen. Channel catfish must be at least 12 inches long to be legal for possession [5].


Sampling and Data Analysis

To evaluate potential health risks to recreational and subsistence fishers who consume environmentallycontaminated seafood, the Texas Department of Health (TDH) collects and analyzes seafood samplesfrom the state's public waters. These samples are representative of available species, trophic levels,lipid content, and legal sizes. When it is appropriate and practical, TDH collects samples from severallocations within a water body to characterize the distribution of contaminants in seafood from that waterbody. It is important to note that people eating contaminated seafood are most likely exposed over thelong term through consumption of one or more species contaminated with low concentrations ofenvironmental pollutants. Consequently, people exposed to environmental contaminants throughconsumption of seafood are unlikely to display acute or overt toxicity. Instead, subtle, delayed, orchronic adverse health effects may be more commonly expected.

Thus, the main purpose of TDH contaminant studies is to examine human exposure to speciescommonly consumed over time. TDH typically uses average concentrations of chemical contaminantsacross species and/or sites to assess the probability of adverse health outcomes from low-level, long-termexposure. Despite the possibility that using average concentrations to estimate risk may lead to over-or underestimates of individual point-in-time exposures, use of averages is a reasonable approach topredicting long-term exposure to low levels of contaminants. Although TDH uses averageconcentrations to determine exposure doses, the agency may use other statistical procedures to assessthe likelihood of adverse health effects from consumption of contaminated seafood when theseprocedures are appropriate.

The Seafood Safety Division collected seventeen fish samples from Lake Raven in late 2000 (tenlargemouth bass and seven channel catfish). Using established methodology, the TDH laboratoryanalyzed edible fillets (skin off) from one largemouth bass and one channelcatfish for seven metals:arsenic, cadmium, copper, lead, mercury, selenium, and zinc; volatile organic compounds (VOCs);semivolatile organic compounds (SVOCs); pesticides; and polychlorinated biphenyls (PCBs). TheTDH laboratory analyzed fifteen additional samples (nine largemouth bass and six channel catfish) formercury only.

Fifteen of seventeen samples collected from Lake Raven contained detectable levels of mercury (Table1): all largemouth bass and five of seven channel catfish. Two samples also contained low levels of zincand selenium. The laboratory reported no pesticides, VOCs, SVOCs, or PCBs in these samples.

Deriving Health-Based Assessment Comparison Values (HACs)

TDH evaluated chemical contaminants in fish from Lake Raven by comparing average concentrationsof chemical contaminants with health-based assessment comparison (HAC) values for non-cancer andcancer endpoints. TDH used the U.S. Environmental Protection Agency's (USEPA) reference doses(RfDs) or the Agency for Toxic Substances and Disease Registry's (ATSDR) minimal risk levels(MRLs) to derive the noncancer HAC values. RfDs [6] and MRLs are estimates of daily exposures tocontaminants that are unlikely to cause adverse noncancer health effects, even if exposure occurs overa lifetime. The cancer risk comparison values in this health consultation are based on the USEPA'schemical-specific cancer slope factors (SF), an estimated lifetime risk of 1 excess cancer in 10,000 (1x 10-4) people exposed, and an exposure period of 30 years. TDH utilized standard assumptions forbody weight (70 kilograms, adult; 35 kilograms, child) and fish consumption (30 grams per day, adult;15 grams per day, child) to calculate the HAC values [7]. Many of the constants employed incalculating HAC values have margins of safety built into them. Thus, adverse health effects will notnecessarily occur simply because concentrations of toxicants in seafood exceed HAC values. Moreover,health-based assessment comparison values do not represent a sharp dividing line between safe andunsafe exposures. The strict demarcation between acceptable and unacceptable levels of exposure orrisk is primarily a tool used by risk managers to ensure protection of public health. TDH views it asunacceptable when consumption of one or fewer meals per week would result in an exposure that exceeds a HAC value or other measures of risk.

Addressing the Potential for Cumulative Effects

When multiple chemicals affect the same target organ, or when several chemicals present in seafoodcould be carcinogens, TDH assumes that adverse health effects are cumulative (i.e., additive) [8]. Toevaluate the potential public health impact of additive noncancerous health effects, TDH calculates ahazard index (HI), which is the sum of the ratios of the estimated exposure doses for each contaminantdivided by its respective RfD or MRL. A HI of less than 1 suggests that exposure to combinedcontaminants at specified concentrations is unlikely to cause adverse noncancerous health effects, evenif that exposure continues for many years. On the other hand, while a HI greater than 1 does notnecessarily mean exposure to the contaminants will result in adverse health effects, it does suggest thatthe agency might consider some public health intervention. To estimate the potential excess lifetimecancer risk from simultaneous exposure to multiple carcinogens, TDH calculates the cumulative riskby summing the estimated risk for each contaminant. TDH recommends limiting consumption ofseafood contaminated with multiple carcinogenic chemicals to amounts resulting in an estimatedtheoretical lifetime cancer risk of not more than 1 excess cancer in 10,000 persons exposed for a lifetime through consumption of seafood.

Addressing the Unique Vulnerabilities of Children

TDH recognizes that fetuses, infants, and children may be uniquely vulnerable to the effects of toxicchemicals and that any such vulnerabilities demand special attention. Windows of vulnerability, i.e.,critical periods, exist during development. These critical periods are particularly evident during earlygestation, but also appear throughout pregnancy, infancy, childhood, and adolescence - indeed, at anytime when toxicants can permanently impair or alter structure or function [9]. Unique childhoodvulnerabilities may result from the fact that at birth, many organs and body systems, including the lungs,immune, endocrine, reproductive, and nervous systems, have not achieved structural or functionalmaturity; these organ systems continue to develop throughout childhood and adolescence. Children canalso differ from adults in absorption, metabolism, storage, and excretion of toxicants, any of which couldresult in higher biologically effective doses to target organs. Children's exposure to toxicants may bemore extensive than that of adults because children consume more food and liquids in proportion to theirbody weight than do adults [9]. They can also ingest toxicants through breast milk - oftenunrecognized as an exposure pathway. Thus, children may experience toxic effects at a lower exposuredose than would affect adults. Stated differently, children could respond more severely than wouldadults to an equivalent exposure dose [9]. Children may also be more prone than are adults todeveloping certain cancers from chemical exposures. Therefore, in accordance with ATSDR's ChildHealth Initiative [10] and USEPA's National Agenda to Protect Children's Health fromEnvironmental Threats [9], TDH evaluated the potential public health hazards to children who eat fishfrom Lake Raven. Based on this health consultation, TDH has determined that consumption of fish from Lake Raven does not pose a public health hazard to children.

Characterizing the Risk

Assessing the Risk of Noncancerous Health Effects

Mercury was measurable in fifteen of seventeen fish sampled from Lake Raven (Table 1). The averagemercury (Hg) concentration for all samples was 0.332 mg/kg in edible tissue; the average mercuryconcentration in channel catfish was 0.139 mg/kg, while mercury in largemouth bass averaged 0.466mg/kg (95% CI: 0.345 to 0.587). Despite these species differences, the average concentration ofmercury in each species did not exceed the health-based assessment comparison value (HAC value) formethylmercury. Neither did the average mercury concentration for all samples exceed the HAC value. Although the mercury level in largemouth bass is higher than in channel catfish, the mean concentrationin largemouth bass and the predicted concentration at the 95% upper confidence limit on the averageconcentration did not exceed TDH guidelines for mercury. Moreover, state regulations prohibitpossession of largemouth bass from Lake Raven; thus, exposure to exceptional levels of mercury fromthis species is not likely. Channel catfish contained only very low levels of mercury and thus, do not pose a risk to public health.

Assessing the Risk of Cancer

Few published reports exist of cancer in humans after exposure to methylmercury [2]. Althoughmethylmercury has been associated with neoplastic changes in the kidneys of experimental animals,those changes generally occurred only at doses that caused significant systemic toxicity and wereassociated with alterations in structure or function that were classified by investigators as thresholdeffects [2]. Thus, although EPA determined that methylmercury is a possible human carcinogen (GroupC) [2], it is likely that systemic health effects would be observed at methylmercury exposures muchlower than those required for tumor formation. Long-term administration of methylmercury toexperimental animals produces overt symptoms of neurotoxicity at daily doses an order of magnitudelower than those required to induce tumors in mice. Thus, the USEPA has not derived a cancer slopefactor for methylmercury. For this reason, TDH did not assess potential carcinogenic risk from consuming mercury-contaminated fish from Lake Raven.

Assessing cumulative noncancerous and carcinogenic effects

There were no contaminants identified in fish from Lake Raven at levels of concern for cumulativeeffects. Therefore, TDH did not evaluate samples from this water body for potential cumulative adverse health effects.


  1. The average levels of mercury in fish from Lake Raven do not exceed the health-basedassessment comparison value for methylmercury. Neither channel catfish nor largemouth bassfrom Lake Raven contain mercury at concentrations that exceed the HAC value formethylmercury. Thus, consumption of fish from Lake Raven does not pose a public health hazard.


TDH uses established criteria for issuing fish consumption advisories. When analysis confirms thatconsumption of one or fewer meals per week (adults: eight ounces; children: four ounces) would resultin exposures that exceed health-based assessment guidelines established by the department, riskmanagers may recommend that the Commissioner of Health issue a consumption advisory or ban thepossession of fish from a given water body. Based on this health consultation, the Seafood SafetyDivision and the Environmental Epidemiology and Toxicology Division recommend that:

  1. TDH presently takes no public health action for Lake Raven.

  2. TDH continues to monitor mercury levels in fish from Lake Raven as resources allow.


Information about TDH fish consumption advisories and bans is available to the public throughthe TDH Seafood Safety Division (512-719-0215) or on the World Wide Web at Health consultations dealing with contaminants in seafoodfrom Texas waters may also be available to the public from the Agency for Toxic Substancesand Disease Registry ( The TexasDepartment of Health provides this information to the U.S. Environmental Protection Agency(, the Texas Natural Resource Conservation Commission (TNRCC and to the Texas Parks and Wildlife Department (TPWD Each year, the TWPD informs the fishing and hunting public ofclosure areas in an official hunting and fishing regulations booklet [5] that is available at somestate parks and at establishments that sell fishing licenses.

If questions or concerns arise about the scientific information presented in this in this healthconsultation, readers may telephone the Seafood Safety Division (512-719-0215) or theEnvironmental Epidemiology and Toxicology Division (512- 458-7269) at the TexasDepartment of Health. Toxicological information is also available from the Agency for ToxicSubstances and Disease Registry (ATSDR), Division of Toxicology, at the toll-free number(800-447-1544) in Atlanta, Georgia.

Table 1.

Mercury concentrations (mg/kg) in fish taken from Lake Raven in 2000.
Species Number Affected/ Sampled Average Concentration
Health Assessment Comparison Value Basis for Comparison Value
Channel catfish 5/7 0.139 nd-0.232) 0.700 mg/kg ATSDR MRL: 0.0003 mg/kg/day
Largemouth bass 10/10 0.466 (0.203-0.740)
All Species 15/17 0.332 (nd-0.740)

*Minimum concentration to Maximum concentration (to calculate the range, subtract the minimum concentration from the maximum concentration).
Derived from the Minimal Risk Level for noncarcinogens; assumes a body weight of 70 kg, and a consumption rate of 30 grams per day.
nd: not detected at concentrations above the laboratory reporting limit.


  1. Mills, R. [TPWD] Texas Parks and Wildlife Department. Austin, Texas: Personal Communication: 2000.

  2. [IRIS] Integrated risk information system. US Environmental Protection Agency. Office of Research and Development, National Center for Environmental Assessment. Available from URL: .

  3. [USCB] United States Census Bureau. 2000 Decennial Census. 2001. Available from URL:

  4. [TPWD] Texas Parks and Wildlife Department. Austin, Texas: Huntsville State Park. Available from URL:

  5. [TPWD] Texas Parks and Wildlife. Austin, Texas. 2000-2001 outdoor annual: official hunting and fishing regulations. Texas Monthly Custom Publishing: 2000.

  6. [USEPA] U.S. Environmental Protection Agency. Glossary of risk assessment-related terms. Integrated Risk Information System (IRIS): 1994.

  7. [USEPA] U.S. Environmental Protection Agency. Guidance for assessing chemical contaminant data for use in fish advisories, Volume 2, risk assessment and fish consumption limits, 3rd ed. Washington D.C: 2000 August.

  8. [USEPA] U.S. Environmental Protection Agency. Guidelines for the health risk assessment of chemical mixtures. Washington D.C: 1986.

  9. [USEPA] U.S. Environmental Protection Agency. Office of Research and Development. Strategy for research on environmental risks to children section, 1.2. Washington, D.C: 2000 August.

  10. [ATSDR] Agency for Toxic Substances and Disease Registry. Office of Children's Health. Child health initiative. Atlanta: US Department of Health and Human Services: 1995.


Jerry Ann Ward, Ph.D.
Seafood Safety Division
Bureau of Food and Drug Safety

G. Kirk Wiles
Seafood Safety Division
Bureau of Food and Drug Safety

Eric Fonken, D.V.M., M.P.Aff.
Assistant Director
Seafood Safety Division
Bureau of Food and Drug Safety

Susan Bush, B.S.
Survey Branch Chief
Seafood Safety Division
Bureau of Food and Drug Safety

Lisa Williams, M.S.
Environmental Epidemiology and Toxicology Division
Bureau of Epidemiology

John F. Villanacci, Ph.D.
Environmental Epidemiology and Toxicology Division
Bureau of Epidemiology


George Pettigrew, P.E.
Senior Regional Representative
ATSDR - Region 6


Alan W. Yarbrough
Environmental Health Scientist
Division of Health Assessment and Consultation
Superfund Site Assessment Branch
State Programs Section


This Lake Raven Health Consultation was prepared by the Texas Department of Health under a cooperative agreement with the Agency for Toxic Substances and Disease Registry (ATSDR). It is in accordance with approved methodology and procedures existing at the time the health consultation was initiated.

Alan W. Yarbrough
Technical Project Officer, SPS, SSAB, DHAC, ATSDR

The Division of Health Assessment and Consultation, ATSDR, has reviewed this healthconsultation and concurs with its findings.

Lisa C. Hayes
for Chief, State Programs Section, SSAB, DHAC, ATSDR

Table of Contents The U.S. Government's Official Web PortalDepartment of Health and Human Services
Agency for Toxic Substances and Disease Registry, 4770 Buford Hwy NE, Atlanta, GA 30341
Contact CDC: 800-232-4636 / TTY: 888-232-6348

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