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Report on the Environment

Consumption of Fish and Shellfish

What are the trends in the condition of consumable fish and shellfish and their effects on human health?

Fish and shellfish caught through commercial, recreational, or subsistence fishing are an important part of a healthful diet for many people. Fish and shellfish contain high-quality protein and other essential nutrients, are low in saturated fat, and contain omega-3 fatty acids. Most fish consumed in the United States comes from commercial fisheries, and is purchased in supermarkets or fish markets. Fishing also is one of the most popular outdoor recreational activities in the country, with more than 34 million people per year fishing recreationally30—many of whom eat at least some of the fish they catch. In addition, subsistence fishers—people who rely on fish as an affordable food source or for whom fish are culturally important—consume fish and shellfish as a major part of their diets. Commercial, recreational, and subsistence fisheries all have substantial economic value for the nation, regions, and local communities.

Americans consume fish and shellfish caught in the nation’s lakes, rivers, and estuaries and in deep ocean fisheries, as well as farmed fish and shellfish.31 Some of these fish and shellfish contain elevated levels of chemical or biological contaminants. This question addresses the condition of consumable fish and shellfish caught or farmed in the United States—whether, and the extent to which, these organisms contain contaminants that could affect the health of people who consume them.

According to recent surveys, the average American consumes close to 13 grams of fish and shellfish per day (prepared weight), which amounts to slightly more than one 3-ounce serving per week.32 However, many Americans consume substantially more fish and shellfish than the national average; some of the highest consumption rates are among tribal and ethnic populations who fish for subsistence. Concern about fish and shellfish safety is higher for these groups as well as for children, pregnant and nursing women (because of possible effects on the fetus or infant), and other population subgroups who may be more vulnerable to the health effects of certain chemical or biological contaminants (e.g., elderly or immunosuppressed individuals).

Chemical contaminants of greatest concern in consumable fish and shellfish tend to be those that are persistent, bioaccumulative, and toxic (called PBTs). These chemicals can persist for long periods in sediments and then enter the food web when ingested by bottom-dwelling (benthic) organisms. Benthic organisms are eaten by small fish, which in turn are eaten by larger fish, which may be consumed by humans or wildlife. PBTs that are common in fresh and coastal waters include:

  • Mercury. This highly toxic metal is present in waters all over the globe—a result of long-range transport and deposition of airborne mercury as well as direct inputs to water.33 Mercury in water bodies can be methylated by certain bacteria in bottom sediments to form methylmercury, which is more toxic and bioavailable than other forms of mercury.34 It also is biomagnified through aquatic food webs, so that it becomes particularly concentrated in larger and longer-lived predators such as bass, tuna, swordfish, and some sharks. Exposure to high levels of methylmercury can cause reproductive and other effects in wildlife;35 in humans, exposure to elevated levels is primarily associated with developmental and neurological health effects.36

  • Polychlorinated biphenyls (PCBs) and the pesticide DDT. Though PCBs and DDT are no longer manufactured or used in the U.S., they persist in historical deposits in watersheds and near-shore sediments, which can continue to contaminate fish and shellfish. These chemicals are also circulated globally as a result of use in other parts of the world. Levels of PCBs and DDT are a concern in some bottom-feeding fish and shellfish, as well as in some higher-level predators. These chemicals have been linked to adverse health effects such as cancer, nervous system damage, reproductive disorders, and disruption of the immune system in both humans and wildlife.

Other chemical contaminants that may be present in fish and shellfish include other pesticides, metals (such as arsenic), and dioxins and furans.37

Biological contamination also can affect the condition of fish and shellfish—particularly the latter. For example, shellfish contaminated with pathogens from human and animal fecal wastes can cause gastrointestinal illness and even death in individuals with compromised immune systems. Sources of fecal contamination in shellfish include urban runoff, wildlife, wastewater treatment systems and treatment plants, agricultural runoff, and boating and marinas.

Marine biotoxins produced by certain types of algae can contaminate fish and shellfish as well. These toxins not only can harm fish and fish communities—sometimes resulting in massive fish kills or losses to aquaculture operations—but they also can make their way through the food web to affect seabirds, marine mammals, and humans. Mollusks such as mussels, clams, oysters, whelks, and other shellfish can carry biotoxins that have common symptoms such as irritation of the eyes, nose, throat, and tingling of the lips and tongue. Consumption of contaminated seafood can cause a range of other health effects in humans, depending on the organism involved, including gastrointestinal illness, amnesia, memory loss, paralysis, and even death.38,39

The growth of aquaculture, or fish farming, may affect the levels of certain contaminants in consumable fish and shellfish. Dense colonies can increase stress and disease transmission among fish, in some cases requiring the administration of antibiotics.40 Studies have also found higher levels of certain contaminants in farmed fish than in their wild counterparts, possibly due to differences in diet. For example, several studies have found higher concentrations of PCBs, organochlorine pesticides, and polybrominated diphenyl ethers (PBDEs) in farmed salmon.41

Overharvesting also can affect the condition of fish and shellfish—not only the species being harvested, but also the species that prey on them—by disrupting the food web. Because of depleted food sources, predators can become more susceptible to disease (such as infection of rockfish by mycobacterial lesions). These infections are often confined to internal organs and may not be apparent to anglers, although in some cases they are associated with external sores as well. Some types of mycobacteria can also infect humans who handle diseased fish if the infection comes into contact with an open wound. The slow-developing infections are usually not severe in humans, but in some cases they can cause major health problems, especially in people with compromised immune systems.

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