Final Report: The Role of Fishes as Transporters of Mercury

EPA Grant Number: R825433C012
Subproject: this is subproject number 012 , established and managed by the Center Director under grant R825433
(EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).

Center: EERC - Center for Ecological Health Research (Cal Davis)
Center Director: Rolston, Dennis E.
Title: The Role of Fishes as Transporters of Mercury
Investigators: Cech, Joseph
Institution: University of California - Davis
EPA Project Officer: Packard, Benjamin H
Project Period: October 1, 1996 through September 30, 2000
RFA: Exploratory Environmental Research Centers (1992) RFA Text |  Recipients Lists
Research Category: Center for Ecological Health Research , Targeted Research

Objective:

The objective of this project was to determine methylmercury (MeHg) uptake rates and bioenergetic effects in adult and juvenile Sacramento blackfish (Orthodon microlepidotus); a native, planktivorous species that is commercially fished in Clear Lake, California. Clear Lake has a U.S. Environmental Protection Agency Superfund site, the Sulphur Bank Mercury Mine, at one end and high levels of mercury, including MeHg, in sediments and biota (especially near the mine site).

Summary/Accomplishments (Outputs/Outcomes):

We made the first, exhaustive study of transgill MeHg uptake by fish, and found that a large percentage (35.6 percent) of environmentally relevant concentrations (1.4 ng/L) of MeHg is taken up by Sacramento blackfish (O. microlepidotus) with a single, ventilatory pass over the gills. However, this high uptake rate is significantly decreased by the presence of dissolved organic carbon (DOC), a mercury-complexing agent, in the water. Specifically, 2 and 5 mg carbon/L decreased the mean MeHg extraction efficiency by 78 percent and 85 percent, respectively. These results emphasize the futility of taking water samples from natural environments and assuming that all of the measured MeHg will be available to the resident fishes.

We also found that the presence of 2-5 mg/L DOC significantly decreased MeHg accumulation in the gills, kidney, and spleen of Sacramento blackfish. Thus, mercury concentrations in fish tissues taken from aquatic systems may reflect the influence of DOC (or other complexing agents), as well as mercury concentrations in these systems.

In conducting these experiments, we found unexpectedly high mercury levels in pelleted commercial fish feeds. This information should be considered by investigators using fish as models for mercury exposure and effects. Because of this finding, "control" fish ingesting "mercury-free" diets or being held in mercury-free water can be expected to take up mercury from commercial diets used to maintain the fish in good condition.

Our study is the first to show the dose-dependent effects of dietary MeHg exposure on fish bioenergetics. High (55.5 mg/kg food) and medium (22.2 mg/kg) MeHg concentrations decreased juvenile Sacramento blackfish growth rates, as compared with fish that were fed lower concentrations. Furthermore, high-dose fish experienced significantly decreased mean gross conversion efficiency, as compared with controls. In contrast, wet/dry muscle mass ratio, muscle mass/total mass ratio, condition factor (K, weight/length3), and resting routine oxygen consumption rates were not statistically distinguishable among the dose (treatment) groups. In addition, we were the first investigators to determine the long-term (weeks) dietary MeHg assimilation rate of a fish. Juvenile Sacramento blackfish assimilated between 40 and 60 percent of the dietary MeHg fed by 35 days, and between 33-43 percent by 70 days.

Finally, we were the first to determine that dietary MeHg exposure decreases critical swimming velocity and optomotor responses in a fish. Sacramento blackfish exposed to a high (55.5 mg/kg food) MeHg dose showed decreased swimming performance, as compared with the other treatments. Our findings build on earlier research of mercury's known neurotoxic effects on fish locomotory functions (including possible effects on foraging, antipredator behavior, and migration) and possible sensory functions (orientation loss in high-dose, optomotor experiment fish). Although the effect thresholds may differ among fish species, these results contribute to improved, ecosystem-based mercury-movement models, a deeper understanding of MeHg-related effects on fish and fish populations, and more accurate fish population estimates—all of which allow better informed natural resource management at Clear Lake and other aquatic systems.

The following activities were accomplished:

• We found unexpectedly high mercury levels in pelleted, commercial fish feeds. Thus, control fish ingesting mercury-free diets or being held in mercury-free water can be expected to take up mercury from commercial diets used to maintain the good condition of the fish. This information should be used by investigators working with fish as models for mercury exposure and effects.

• We made the first, exhaustive study of transgill MeHg uptake by fish. We found that a large percentage (35.6 %) of environmentally relevant concentrations (1.4 ng/L) of MeHg is taken up by Sacramento blackfish (O. microlepidotus) with a single, ventilatory pass over the gills. However, this high uptake rate is significantly decreased by the presence of DOC as a complexing agent. Specifically, 2 and 5 mg carbon/L decreased the mean MeHg extraction efficiency by 78 percent and 85 percent, respectively. These results emphasize the futility of taking water samples from natural environments and assuming that all of the measured MeHg will be available to the resident fish. Thus, these results contribute to the construction of better ecosystem-based mercury-movement models, and to more informed natural resource management at Clear Lake and other aquatic systems.

• We found that the presence of 2-5 mg/L DOC significantly decreases mercury accumulation in the gills, kidney, and spleen of Sacramento blackfish. Thus, mercury concentrations in fish tissues taken from aquatic systems may reflect the influence of DOC (or other complexing agents), as well as mercury concentrations in these systems. These results contribute to the construction of better ecosystem-based mercury-movement models, and to more informed natural resources management at Clear Lake and other aquatic systems.

• Our results are the first to show the dose-dependent effects of dietary MeHg exposure on fish bioenergetics. High (55.5 mg/kg food) and medium (22.2 mg/kg) MeHg concentrations decreased Sacramento blackfish growth rates, as compared with fish that were fed lower concentrations. Furthermore, high-dose fish experienced significantly decreased mean gross conversion efficiency, as compared with controls. In contrast, wet/dry muscle mass ratio, muscle mass/total mass ratio, condition factor (K, weight/length3), and resting routine oxygen consumption rates were not statistically distinguishable among treatment groups. Thus, these results contribute to a better understanding of MeHg-related effects on fish and fish populations, leading to better fish population estimates and more informed natural resource management at Clear Lake and other aquatic systems.

• Our study is the first to determine the long-term (weeks) dietary MeHg assimilation rate of a fish. Juvenile Sacramento blackfish assimilated between 40 and 60 percent of the dietary MeHg fed by 35 days, and between 33-43 percent by 70 days. These results contribute to the construction of better ecosystem-based mercury-movement models, and to more informed natural resource management at Clear Lake and other aquatic systems.

• We determined, for the first time, that dietary MeHg exposure decreases critical swimming velocity and optomotor responses in a fish. Sacramento blackfish exposed to a high (55.5 mg/kg food) MeHg dose showed decreased swimming performance, as compared with the other treatments. These results are important because they add to the research on mercury's known neurotoxic effects on fish locomotory functions (including possible effects on foraging, antipredator behavior, and migration) and possible sensory functions (orientation loss in high-dose, optomotor experiment fish). Although the effects threshold may differ among fish species, these results contribute to a better understanding of MeHg-related effects on fish and fish populations, leading to better fish population estimates and more informed natural resource management at Clear Lake and other aquatic systems.

• In general, our study has narrowed the gap between environmental and experimental observations of mercury contamination in fish, and has provided much needed information to improve MeHg modeling and risk assessment efforts for wildlife and humans.

Supplemental Keywords:

ecosystem, ecosystem protection, environmental exposure and risk, geographic area, international cooperation, water, terrestrial ecosystems, aquatic ecosystem, aquatic ecosystem restoration, aquatic ecosystems and estuarine research, biochemistry, ecological effects, ecological indicators, ecological monitoring, ecology and ecosystems, environmental chemistry, restoration, state, water and watershed, watershed, watershed development, watershed land use, watershed management, watershed modeling, watershed restoration, watershed sustainability, agricultural watershed, exploratory research environmental biology, California, CA, Clear Lake, Lake Tahoe, anthropogenic effects, aquatic habitat, biogeochemical cycling, ecological assessment, ecology assessment models, ecosystem monitoring, ecosystem response, ecosystem stress, environmental stress, environmental stress indicators, fish habitat, hydrologic modeling, hydrology, integrated watershed model, lake ecosystems, lakes, land use, nutrient dynamics, nutrient flux, water management options, water quality, wetlands., RFA, Health, Scientific Discipline, PHYSICAL ASPECTS, Water, INDUSTRY, Ecosystem Protection/Environmental Exposure & Risk, HUMAN HEALTH, Aquatic Ecosystems & Estuarine Research, mercury transport, Health Risk Assessment, Exposure, Aquatic Ecosystem, Risk Assessments, Biochemistry, Physical Processes, Ecological Risk Assessment, Industrial Processes, Biology, Mercury, fate and transport, marine food web, mercury uptake, chemical exposure, contaminant exposure, human consumption, food web, food chain, Clear Lake , Sacramento blackfish, bioavailability, marine biology, fish consumption, fish communities, Clear Lake, human exposure, methylmercury, mining, watershed influences, aquatic ecosystems, mercury contamination in fish, mercury methylation, dietary exposure, bioaccumulation, fish-borne toxicants, human health risk, lake ecosystems

Relevant Websites:

http://ice.ucdavis.edu/cehr/ Exit

Progress and Final Reports:

Original Abstract
  • 1997
  • 1998
  • 1999

  • Main Center Abstract and Reports:

    R825433    EERC - Center for Ecological Health Research (Cal Davis)

    Subprojects under this Center: (EPA does not fund or establish subprojects; EPA awards and manages the overall grant for this center).
    R825433C001 Potential for Long-Term Degradation of Wetland Water Quality Due to Natural Discharge of Polluted Groundwater
    R825433C002 Sacramento River Watershed
    R825433C003 Endocrine Disruption in Fish and Birds
    R825433C004 Biomarkers of Exposure and Deleterious Effect: A Laboratory and Field Investigation
    R825433C005 Fish Developmental Toxicity/Recruitment
    R825433C006 Resolving Multiple Stressors by Biochemical Indicator Patterns and their Linkages to Adverse Effects on Benthic Invertebrate Patterns
    R825433C007 Environmental Chemistry of Bioavailability in Sediments and Water Column
    R825433C008 Reproduction of Birds and mammals in a terrestrial-aquatic interface
    R825433C009 Modeling Ecosystems Under Combined Stress
    R825433C010 Mercury Uptake by Fish
    R825433C011 Clear Lake Watershed
    R825433C012 The Role of Fishes as Transporters of Mercury
    R825433C013 Wetlands Restoration
    R825433C014 Wildlife Bioaccumulation and Effects
    R825433C015 Microbiology of Mercury Methylation in Sediments
    R825433C016 Hg and Fe Biogeochemistry
    R825433C017 Water Motions and Material Transport
    R825433C018 Economic Impacts of Multiple Stresses
    R825433C019 The History of Anthropogenic Effects
    R825433C020 Wetland Restoration
    R825433C021 Sierra Nevada Watershed Project
    R825433C022 Regional Transport of Air Pollutants and Exposure of Sierra Nevada Forests to Ozone
    R825433C023 Biomarkers of Ozone Damage to Sierra Nevada Vegetation
    R825433C024 Effects of Air Pollution on Water Quality: Emission of MTBE and Other Pollutants From Motorized Watercraft
    R825433C025 Regional Movement of Toxics
    R825433C026 Effect of Photochemical Reactions in Fog Drops and Aerosol Particles on the Fate of Atmospheric Chemicals in the Central Valley
    R825433C027 Source Load Modeling for Sediment in Mountainous Watersheds
    R825433C028 Stress of Increased Sediment Loading on Lake and Stream Function
    R825433C029 Watershed Response to Natural and Anthropogenic Stress: Lake Tahoe Nutrient Budget
    R825433C030 Mercury Distribution and Cycling in Sierra Nevada Waterbodies
    R825433C031 Pre-contact Forest Structure
    R825433C032 Identification and distribution of pest complexes in relation to late seral/old growth forest structure in the Lake Tahoe watershed
    R825433C033 Subalpine Marsh Plant Communities as Early Indicators of Ecosystem Stress
    R825433C034 Regional Hydrogeology and Contaminant Transport in a Sierra Nevada Ecosystem
    R825433C035 Border Rivers Watershed
    R825433C036 Toxicity Studies
    R825433C037 Watershed Assessment
    R825433C038 Microbiological Processes in Sediments
    R825433C039 Analytical and Biomarkers Core
    R825433C040 Organic Analysis
    R825433C041 Inorganic Analysis
    R825433C042 Immunoassay and Serum Markers
    R825433C043 Sensitive Biomarkers to Detect Biochemical Changes Indicating Multiple Stresses Including Chemically Induced Stresses
    R825433C044 Molecular, Cellular and Animal Biomarkers of Exposure and Effect
    R825433C045 Microbial Community Assays
    R825433C046 Cumulative and Integrative Biochemical Indicators
    R825433C047 Mercury and Iron Biogeochemistry
    R825433C048 Transport and Fate Core
    R825433C049 Role of Hydrogeologic Processes in Alpine Ecosystem Health
    R825433C050 Regional Hydrologic Modeling With Emphasis on Watershed-Scale Environmental Stresses
    R825433C051 Development of Pollutant Fate and Transport Models for Use in Terrestrial Ecosystem Exposure Assessment
    R825433C052 Pesticide Transport in Subsurface and Surface Water Systems
    R825433C053 Currents in Clear Lake
    R825433C054 Data Integration and Decision Support Core
    R825433C055 Spatial Patterns and Biodiversity
    R825433C056 Modeling Transport in Aquatic Systems
    R825433C057 Spatial and Temporal Trends in Water Quality
    R825433C058 Time Series Analysis and Modeling Ecological Risk
    R825433C059 WWW/Outreach
    R825433C060 Economic Effects of Multiple Stresses
    R825433C061 Effects of Nutrients on Algal Growth
    R825433C062 Nutrient Loading
    R825433C063 Subalpine Wetlands as Early Indicators of Ecosystem Stress
    R825433C064 Chlorinated Hydrocarbons
    R825433C065 Sierra Ozone Studies
    R825433C066 Assessment of Multiple Stresses on Soil Microbial Communities
    R825433C067 Terrestrial - Agriculture
    R825433C069 Molecular Epidemiology Core
    R825433C070 Serum Markers of Environmental Stress
    R825433C071 Development of Sensitive Biomarkers Based on Chemically Induced Changes in Expressions of Oncogenes
    R825433C072 Molecular Monitoring of Microbial Populations
    R825433C073 Aquatic - Rivers and Estuaries
    R825433C074 Border Rivers - Toxicity Studies