EPA-Expo-Box (A Toolbox for Exposure Assessors)
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Aquatic biota (i.e., fish or shellfish) may be exposed to contaminants through direct uptake from water through gills or other biological membranes (bioconcentration) and by uptake through ingestion of contaminated plants or animals (bioaccumulation), and these contaminants have the potential to biomagnify up the food chain. The concentration of a contaminant in a particular species to which a human consumer is potentially exposed depends on how much of the chemical the species has accumulated during its lifetime—and this will be dictated by factors related to the chemical itself (physicochemical factors and where it accumulates in the body) and the animal (species, size, and age). A consumer’s cooking and preparation methods may affect the contaminant mass to which he/she might be exposed.
Physicochemical factors can affect the bioavailability of the contaminant to aquatic biota. These include solubility and partition coefficients that dictate whether the chemical will be available for uptake from the water column (through consumption or direct transfer from the water) or from aquatic sediments. The Kow is correlated with the potential for a chemical to bioaccumulate in the fat of organisms, and contaminants with a Kow value that is ≥4.2 are considered to have high bioaccumulation potential (like many PBTs).
Some contaminants (e.g., dioxins/PCBs, pesticides) are lipophilic and tend to concentrate in the fatty tissues of aquatic biota. PCBs also concentrate in heads, guts, and liver. Other contaminants, like methyl mercury, accumulate in the muscle and other living tissue of aquatic organisms (e.g., edible fish fillet). Mercury in water bodies can be methylated by certain bacteria in bottom sediments making bottom-dwelling aquatic biota susceptible to direct exposures.
Important fish and shellfish factors include species type and lipid levels; metabolism rate (which is affected by the animal’s size, health condition, and activity level); and trophic level (effect of biomagnification). Fish and shellfish size and age are also important factors in contaminant accumulation. For example, larger size classes of fish within the same species generally contain higher concentrations of bioaccumulative contaminants because larger fish are older, they have had more time to accumulate chemicals from their food, and they are more likely to catch larger prey, which themselves have had a longer time to bioaccumulate chemicals. Older fish also concentrate more contaminants in their muscle tissues, which are fattier than muscle tissue in younger fish.
Other factors that impact exposure concentration are food preparation and cooking methods. Most fishers in the United States consume fish fillets. However, certain populations (e.g., some Asian-Americans, Native American groups) eat parts of the fish other than the fillet and sometimes the whole fish (including skin, heads, fat, viscera), which might contain high levels of some contaminants.
What about Aquaculture?
In the United States, over 100 species of aquatic organisms at different life stages, such as catfish, shrimp, salmon, scallops, oysters, and trout, are produced through aquaculture. How are contaminant levels in these fish and shellfish different from the wild-caught species? Farm-raised does not mean contaminant free. There are numerous studies that have documented the presence of comparable or higher levels of contaminants in farm-raised versus wild-caught fish and shellfish.
With shellfish, such as oysters, snails, and mussels, whole-body consumption frequently occurs even within the general population. Therefore, for most consumers of shellfish, consumption likely means ingestion of the total burden of bioaccumulative contaminants (U.S. EPA, 1983).
As mentioned above, some contaminants tend to concentrate in the fatty tissue of aquatic biota, but some can be reduced by removing organs (e.g., liver) and trimming off the fat, skin, and viscera (i.e., preparation) and by cooking (and draining the fat). For others like methyl mercury that accumulate in the muscle and other tissues of the fillet, contaminant levels cannot be reduced by trimming and cooking (U.S. EPA, 2000b). (Also see http://www.state.nj.us/dep/dsr/fishadvisories/cook-prep.htm.)
Fish consumption advisories are another important factor affecting human exposure to contaminants in fish because they provide warnings to human consumers to avoid consumption of certain kinds or certain amounts of fish contaminated by PBTs and other toxic chemicals. If consumers heed these warnings, then their exposure to contaminant concentrations in these fish will be reduced. PBTs such as mercury, PCBs, dioxins, and pesticides such as DDT are responsible for most national fish consumption advisories.
Fish consumption advisories are compiled in EPA’s Fish Consumption Advisories website, which describes state, tribal, and federally- issued fish and wildlife consumption advisories in the United States and U.S. territories as well as the Canadian provinces and territories. In addition, EPA's Fish Consumption Advisory website provides a compilation of information on locally issued fish advisories and safe eating guidelines and includes access to the National Listing of Fish Advisories (NLFA), an interactive mapping and search tool that can be used to view information about fish advisories based on the geographic location of a water body, the species of the fish, the chemical contaminants identified in the advisory, and the portion of the consumer population for whom the advisory was issued.
NLFA is searchable by state or U.S. territory and water body. Consumption restrictions are indicated as red (fish advisory), purple (safe eating guidelines with some restrictions), or green (safe eating guidelines with no restrictions). Detailed advisory reports provide species of fish, the affected population of consumers, and which contaminant(s) triggered the advisory. State environmental programs and public health departments also issue fish consumption advisories for their state waterbodies. Links to state fish advisory websites and state contact information are available from the NLFA webpage. NLFA information is updated about every two years, so the complete and most up-to-date information should be obtained from the state websites.
Concentrations of contaminants in aquatic biota may be determined based on measurements or modeling. Characterizing contaminant concentrations for an exposure scenario is typically accomplished using some combination of the following approaches:
- Sampling fish or other aquatic biota and measuring contaminant concentrations in tissues
- Modeling the concentrations based on source strength, media transport, and chemical transformation processes
- Using existing, available measured concentration data collected for related analysis or compiled in databases
EPA provides information on measuring or modeling aquatic biota concentrations and on available monitoring data. Information on sampling methods is available to support the measurement of contaminants in aquatic biota. In the absence of monitoring data, a variety of models can be used to estimate contaminant concentrations in aquatic biota based on contaminant transport from other media such as surface water and sediment.
A determination of the concentration of contaminants in aquatic media may be based on measurements or modeling. Information on sampling techniques or protocols and analytical methods can be used to support the measurement of contaminants in aquatic biota.
In the absence of monitoring data, models may be used to estimate the concentrations of contaminants in water and aquatic biota, such as the following.
There are a number of information sources that provide monitoring data on contaminant concentrations in surface water, sediment, and aquatic biota, including the following.