Fish and shellfish are an important part of a healthful diet for many people. Fishing is one of the nation’s most popular recreational activities, and fisheries provide substantial economic value for the nation, regions, and local communities. People consume fish and shellfish caught in the nation’s lakes, rivers, and estuaries and in deep ocean fisheries. They also eat farmed fish and shellfish.
Most fish consumed in the United States comes from commercial fisheries. Many recreational fishers eat at least some of the fish they catch. 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.
Several factors affect the condition of fish and shellfish, including chemical and biological contamination, aquaculture, and overharvesting.
- Chemical contaminants of greatest concern tend to be those that are persistent, bioaccumulative, and toxic (called PBTs). PBTs that are common in fresh and coastal waters include:
- Mercury. This highly toxic metal is present in waters all over the globe as a result of long-range transport and deposition of airborne mercury as well as direct inputs to water. It accumulates in the food chain and becomes concentrated at dangerous levels in larger and longer-lived predators such as bass, tuna, swordfish, and some sharks.
Mercury can be methylated by certain bacteria in bottom sediments to form methylmercury, which is more toxic and bioavailable than other forms of mercury. High levels of methylmercury can cause reproductive and other effects in wildlife.1 In humans, exposure to elevated levels is primarily associated with developmental and neurological effects.2
- Polychlorinated biphenyls (PCBs) and the pesticide DDT. Though no longer made in the United States, these chemicals persist in watersheds and near-shore sediments and can continue to contaminate fish and shellfish.
These chemicals are also circulated globally as a result of use in other parts of the world. They have been linked to health effects such as cancer, nervous system damage, reproductive disorders, and disruption of the immune system in both humans and wildlife.
- Mercury. This highly toxic metal is present in waters all over the globe as a result of long-range transport and deposition of airborne mercury as well as direct inputs to water. It accumulates in the food chain and becomes concentrated at dangerous levels in larger and longer-lived predators such as bass, tuna, swordfish, and some sharks.
- Biological contamination can affect the condition of fish and shellfish—especially shellfish. Eating shellfish contaminated with pathogens from human and animal fecal waste can cause gastrointestinal illness and even death in people with compromised immune systems. Sources of fecal contamination include urban runoff, wildlife, wastewater treatment plants, agricultural runoff, and boating and marinas.
Marine biotoxins produced by certain types of algae can also contaminate fish and shellfish. These toxins can cause massive fish kills and can affect seabirds, marine mammals, and humans. Consuming seafood contaminated with biotoxins can cause gastrointestinal illness, amnesia, memory loss, paralysis, and even death.
- Aquaculture (“fish farming”) can affect the levels of certain contaminants in consumable fish and shellfish. Dense colonies can increase stress and disease transmission, in some cases requiring the use of antibiotics.3 Several studies have found higher levels of certain contaminants in farmed fish compared with wild fish.4
- Overharvesting can affect the condition of fish and shellfish by disrupting the food web. Because of depleted food sources, predators can become more susceptible to disease. If people handle diseased fish, they can become infected, a problem especially for people with compromised immune systems.
Concern about fish and shellfish safety is higher for subsistence fishers, children, pregnant and nursing women (because of possible effects on the fetus or infant), and other groups who may be more vulnerable (such as the elderly or immunosuppressed individuals).
Further, trends in the number of advisories over time may reflect changes in the frequency and intensity of monitoring.5 Thus, advisories cannot provide a consistent national ROE indicator of trends in the condition of consumable fish and shellfish.
References
[1] U.S. Environmental Protection Agency. 1997. Mercury study report to Congress. Volume V: Health effects of mercury and mercury compounds. EPA/452/R-97/007.
[2] National Research Council. 2000. Toxicological effects of methylmercury. Washington, DC: National Academies Press.
[3] Barton, B.A., and G.K. Iwama. 1991. Physiological changes in fish from stress in aquaculture with emphasis of the response and effects of corticosteroids. Annu. Rev. Fish Dis. 1:3-26.
[4] Easton, M.D.L., D. Luszniak, and E. Von der Geest. 2002. Preliminary examination of contaminant loadings in farmed salmon, wild salmon and commercial salmon feed. Chemosphere 46(7):1053-1074.
[5] U.S. Environmental Protection Agency. 2011. Summary of Responses to the 2010 National Survey of Fish Advisory Programs. EPA/820/R-12/001.
